Flutterのビルドが異常に遅くなる

# 概要
Dartパッケージの[cloud_firestore](https://pub.dev/packages/cloud_firestore)を入れたことで、iosのビルドに異常に時間がかかるようになってしまった。（というか、一生ビルドが終わらない、、）

# 解決方法
[Firestore iOS SDK](https://github.com/invertase/firestore-ios-sdk-frameworks)を使うことで、ビルドの時間短縮ができる。

1. `ios/PodFile`に以下を追記する
```
target 'Runner' do
  use_frameworks!
  use_modular_headers!
  
  # この１行をを追加する(最新バージョン)
  pod 'FirebaseFirestore', :git => 'https://github.com/invertase/firestore-ios-sdk-frameworks.git', :tag => '9.0.0' 

  flutter_install_all_ios_pods File.dirname(File.realpath(__FILE__))
end
```

2. `flutter clear`を実行
2. pubspec.yamlで`pub get`を実行
2. ビルドを開始する


# 追記
自分の環境だとFirestore iOS SDKのtagが9.0.0だと失敗してしまった。
```
Launching lib/main.dart on iPhone 12 Pro in debug mode...
Running pod install...
CocoaPods' output:
↳
      Preparing

    Analyzing dependencies

    Inspecting targets to integrate
      Using `ARCHS` setting to build architectures of target `Pods-Runner`: (``)

    Finding Podfile changes
      M FirebaseFirestore
      - Flutter
      - cloud_firestore
      - firebase_auth
      - firebase_core
      - firebase_storage
      - image_picker_ios

    Fetching external sources
    -> Pre-downloading: `FirebaseFirestore` from `https://github.com/invertase/firestore-ios-sdk-frameworks.git`, tag `9.0.0`
      > Copying FirebaseFirestore from `/Users/miyamotonanako/Library/Caches/CocoaPods/Pods/External/FirebaseFirestore/d32736d2fefbda490b7c3d56f3ac4f2f` to `Pods/FirebaseFirestore`
    -> Fetching podspec for `Flutter` from `Flutter`
    -> Fetching podspec for `cloud_firestore` from `.symlinks/plugins/cloud_firestore/ios`
    cloud_firestore: Using Firebase SDK version '8.15.0' defined in 'firebase_core'
    -> Fetching podspec for `firebase_auth` from `.symlinks/plugins/firebase_auth/ios`
    firebase_auth: Using Firebase SDK version '8.15.0' defined in 'firebase_core'
    -> Fetching podspec for `firebase_core` from `.symlinks/plugins/firebase_core/ios`
    firebase_core: Using Firebase SDK version '8.15.0' defined in 'firebase_core'
    -> Fetching podspec for `firebase_storage` from `.symlinks/plugins/firebase_storage/ios`
    firebase_storage: Using Firebase SDK version '8.15.0' defined in 'firebase_core'
    -> Fetching podspec for `image_picker_ios` from `.symlinks/plugins/image_picker_ios/ios`

    Resolving dependencies of `Podfile`
      CDN: trunk Relative path: CocoaPods-version.yml exists! Returning local because checking is only performed in repo update
      CDN: trunk Relative path: all_pods_versions_0_3_5.txt exists! Returning local because checking is only performed in repo update
      CDN: trunk Relative path: Specs/0/3/5/Firebase/9.0.0/Firebase.podspec.json exists! Returning local because checking is only performed in repo update
      CDN: trunk Relative path: Specs/0/3/5/Firebase/8.15.0/Firebase.podspec.json exists! Returning local because checking is only performed in repo update
      CDN: trunk Relative path: Specs/0/3/5/Firebase/8.15.0/Firebase.podspec.json exists! Returning local because checking is only performed in repo update
      CDN: trunk Relative path: Specs/0/3/5/Firebase/8.15.0/Firebase.podspec.json exists! Returning local because checking is only performed in repo update
      CDN: trunk Relative path: Specs/0/3/5/Firebase/8.15.0/Firebase.podspec.json exists! Returning local because checking is only performed in repo update
    [!] CocoaPods could not find compatible versions for pod "FirebaseFirestore":
      In Podfile:
        FirebaseFirestore (from `https://github.com/invertase/firestore-ios-sdk-frameworks.git`, tag `9.0.0`)

        cloud_firestore (from `.symlinks/plugins/cloud_firestore/ios`) was resolved to 3.1.15, which depends on
          Firebase/Firestore (= 8.15.0) was resolved to 8.15.0, which depends on
            FirebaseFirestore (~> 8.15.0)

    /usr/local/Cellar/cocoapods/1.11.3/libexec/gems/molinillo-0.8.0/lib/molinillo/resolution.rb:317:in `raise_error_unless_state'
    /usr/local/Cellar/cocoapods/1.11.3/libexec/gems/molinillo-0.8.0/lib/molinillo/resolution.rb:299:in `block in unwind_for_conflict'
    /usr/local/Cellar/cocoapods/1.11.3/libexec/gems/molinillo-0.8.0/lib/molinillo/resolution.rb:297:in `tap'
    /usr/local/Cellar/cocoapods/1.11.3/libexec/gems/molinillo-0.8.0/lib/molinillo/resolution.rb:297:in `unwind_for_conflict'
    /usr/local/Cellar/cocoapods/1.11.3/libexec/gems/molinillo-0.8.0/lib/molinillo/resolution.rb:257:in `process_topmost_state'
    /usr/local/Cellar/cocoapods/1.11.3/libexec/gems/molinillo-0.8.0/lib/molinillo/resolution.rb:182:in `resolve'
    /usr/local/Cellar/cocoapods/1.11.3/libexec/gems/molinillo-0.8.0/lib/molinillo/resolver.rb:43:in `resolve'
    /usr/local/Cellar/cocoapods/1.11.3/libexec/gems/cocoapods-1.11.3/lib/cocoapods/resolver.rb:94:in `resolve'
    /usr/local/Cellar/cocoapods/1.11.3/libexec/gems/cocoapods-1.11.3/lib/cocoapods/installer/analyzer.rb:1078:in `block in resolve_dependencies'
    /usr/local/Cellar/cocoapods/1.11.3/libexec/gems/cocoapods-1.11.3/lib/cocoapods/user_interface.rb:64:in `section'
    /usr/local/Cellar/cocoapods/1.11.3/libexec/gems/cocoapods-1.11.3/lib/cocoapods/installer/analyzer.rb:1076:in `resolve_dependencies'
    /usr/local/Cellar/cocoapods/1.11.3/libexec/gems/cocoapods-1.11.3/lib/cocoapods/installer/analyzer.rb:124:in `analyze'
    /usr/local/Cellar/cocoapods/1.11.3/libexec/gems/cocoapods-1.11.3/lib/cocoapods/installer.rb:416:in `analyze'
    /usr/local/Cellar/cocoapods/1.11.3/libexec/gems/cocoapods-1.11.3/lib/cocoapods/installer.rb:241:in `block in resolve_dependencies'
    /usr/local/Cellar/cocoapods/1.11.3/libexec/gems/cocoapods-1.11.3/lib/cocoapods/user_interface.rb:64:in `section'
    /usr/local/Cellar/cocoapods/1.11.3/libexec/gems/cocoapods-1.11.3/lib/cocoapods/installer.rb:240:in `resolve_dependencies'
    /usr/local/Cellar/cocoapods/1.11.3/libexec/gems/cocoapods-1.11.3/lib/cocoapods/installer.rb:161:in `install!'
    /usr/local/Cellar/cocoapods/1.11.3/libexec/gems/cocoapods-1.11.3/lib/cocoapods/command/install.rb:52:in `run'
    /usr/local/Cellar/cocoapods/1.11.3/libexec/gems/claide-1.1.0/lib/claide/command.rb:334:in `run'
    /usr/local/Cellar/cocoapods/1.11.3/libexec/gems/cocoapods-1.11.3/lib/cocoapods/command.rb:52:in `run'
    /usr/local/Cellar/cocoapods/1.11.3/libexec/gems/cocoapods-1.11.3/bin/pod:55:in `<top (required)>'
    /usr/local/Cellar/cocoapods/1.11.3/libexec/bin/pod:23:in `load'
    /usr/local/Cellar/cocoapods/1.11.3/libexec/bin/pod:23:in `<main>'

Error running pod install
Error launching application on iPhone 12 Pro.
```

tagを`8.15.0`に変更したら無事ビルド完了！