An error occurred while installing sassc (2.4.0), and Bundler cannot continue.を解決した話

Railsアプリ開発で、Bootstrapを追加し`bundle install`したところ、エラーが発生しました。

```
...
Fetching sassc 2.4.0
Installing sassc 2.4.0 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    current directory: /Users/kurokawakazuhiro/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/sassc-2.4.0/ext
/Users/kurokawakazuhiro/.rbenv/versions/2.6.3/bin/ruby -I /Users/kurokawakazuhiro/.rbenv/versions/2.6.3/lib/ruby/2.6.0 -r ./siteconf20210327-6947-1lwawan.rb
extconf.rb
creating Makefile

current directory: /Users/kurokawakazuhiro/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/sassc-2.4.0/ext
make "DESTDIR=" clean
xcrun: error: invalid active developer path (/Library/Developer/CommandLineTools), missing xcrun at: /Library/Developer/CommandLineTools/usr/bin/xcrun

current directory: /Users/kurokawakazuhiro/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/sassc-2.4.0/ext
make "DESTDIR="
xcrun: error: invalid active developer path (/Library/Developer/CommandLineTools), missing xcrun at: /Library/Developer/CommandLineTools/usr/bin/xcrun

make failed, exit code 1

Gem files will remain installed in /Users/kurokawakazuhiro/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/sassc-2.4.0 for inspection.
Results logged to /Users/kurokawakazuhiro/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/extensions/x86_64-darwin-20/2.6.0/sassc-2.4.0/gem_make.out

An error occurred while installing sassc (2.4.0), and Bundler cannot continue.
Make sure that `gem install sassc -v '2.4.0' --source 'https://rubygems.org/'` succeeds before bundling.

In Gemfile:
  bootstrap was resolved to 4.5.3, which depends on
    sassc-rails was resolved to 2.1.2, which depends on
      sassc
```

## 開発環境

- Ruby 2.6.3
- Rails 6.1.3
- Bootstrap 3.4.1

## 解決したコマンド
```
$ xcode-select --install
```

## 発生原因
macOSのアップデートが原因のようです。
昨日まで普通に使えていたコマンドが急に使えなくなると焦りますよね。

## 詰まったポイント
エラーの原因を見誤ってしまったことです。

```
...
An error occurred while installing sassc (2.4.0), and Bundler cannot continue.
Make sure that `gem install sassc -v '2.4.0' --source 'https://rubygems.org/'` succeeds before bundling.
...
```

``gem install sassc -v '2.4.0' --source 'https://rubygems.org/'
``
を実行して成功するか確かめて！とあるので実行すると…

```
$ gem install sassc -v '2.4.0' --source 'https://rubygems.org/'
Failed to load /Users/kurokawakazuhiro/.gemrc because it doesn't contain valid YAML hash
Building native extensions. This could take a while...
ERROR:  Error installing sassc:
        ERROR: Failed to build gem native extension.

    current directory: /Users/kurokawakazuhiro/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/sassc-2.4.0/ext
/Users/kurokawakazuhiro/.rbenv/versions/2.6.3/bin/ruby -I /Users/kurokawakazuhiro/.rbenv/versions/2.6.3/lib/ruby/2.6.0 -r ./siteconf20210327-6685-13b2ch8.rb extconf.rb
creating Makefile

current directory: /Users/kurokawakazuhiro/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/sassc-2.4.0/ext
make "DESTDIR=" clean
xcrun: error: invalid active developer path (/Library/Developer/CommandLineTools), missing xcrun at: /Library/Developer/CommandLineTools/usr/bin/xcrun

current directory: /Users/kurokawakazuhiro/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/sassc-2.4.0/ext
make "DESTDIR="
xcrun: error: invalid active developer path (/Library/Developer/CommandLineTools), missing xcrun at: /Library/Developer/CommandLineTools/usr/bin/xcrun

make failed, exit code 1
...
```

はい。これもエラーになりました。
エラー文をよく読むと、

```
xcrun: error: invalid active developer path (/Library/Developer/CommandLineTools), missing xcrun at: /Library/Developer/CommandLineTools/usr/bin/xcrun
```

このエラーにたどりつき、調べると簡単に解決策にたどりつきました。

## 反省点
エラー文をよく読み、一つの仮説に執着せず、複数の解決策を実行する。
基本的なことですが、ハマってるときは気づけないんですよね。。。

*参照*
[macOSアップデート後の『xcrun: error: invalid active developer path (/Library/Developer/CommandLineTools)...』の対処法](https://qiita.com/nishina555/items/e23d73067a5cac182a63)