WSL1(Ubuntu20.04)にRustをインストールしようとしてエラーが出た時の解決方法

# エラー内容

[公式](https://www.rust-lang.org/ja/learn/get-started)の通りに`curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh` を実行すると以下のエラーが発生

```
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `22`,
 right: `4`', src/libstd/sys/unix/thread.rs:179:21
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `22`,
 right: `4`', src/libstd/sys/unix/thread.rs:179:21
stack backtrace:
   0:     0x7f21efcdaffd - backtrace::backtrace::libunwind::trace::h812748238d609e46
                               at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1:     0x7f21efcdaffd - backtrace::backtrace::trace_unsynchronized::h7c97e818aebf09c8
                               at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2:     0x7f21efcdaffd - std::sys_common::backtrace::_print_fmt::h60d914263b0ccd71
                               at src/libstd/sys_common/backtrace.rs:78
   3:     0x7f21efcdaffd - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hf78227137afc7565
                               at src/libstd/sys_common/backtrace.rs:59
   4:     0x7f21ef96524c - core::fmt::write::h543cdf60775f89bf
                               at src/libcore/fmt/mod.rs:1069
   5:     0x7f21efcda904 - std::io::Write::write_fmt::h0c7f3ce24c679426
                               at src/libstd/io/mod.rs:1504
   6:     0x7f21efcda105 - std::sys_common::backtrace::_print::h80e55e24be231368
                               at src/libstd/sys_common/backtrace.rs:62
   7:     0x7f21efcda105 - std::sys_common::backtrace::print::h3b197b9c1261c865
                               at src/libstd/sys_common/backtrace.rs:49
   8:     0x7f21efcda105 - std::panicking::default_hook::{{closure}}::ha6c807149ce20f8f
                               at src/libstd/panicking.rs:198
   9:     0x7f21efcd98f4 - std::panicking::default_hook::he49a9c12e358cc45
                               at src/libstd/panicking.rs:218
  10:     0x7f21efcd93b6 - std::panicking::rust_panic_with_hook::h93f74f5ef2f71f31
                               at src/libstd/panicking.rs:515
  11:     0x7f21efcd9198 - rust_begin_unwind
                               at src/libstd/panicking.rs:419
  12:     0x7f21efcd9140 - std::panicking::begin_panic_fmt::hfa6ef29ba81f400e
                               at src/libstd/panicking.rs:373
  13:     0x7f21efc11671 - <rustup::diskio::threaded::Threaded as rustup::diskio::Executor>::join::habf6ee901a47a8a4
  14:     0x7f21efc10c78 - core::ptr::drop_in_place::h0b1b0dcb4b07c267
  15:     0x7f21efb76c40 - core::ptr::drop_in_place::h09572fbec04e583f
  16:     0x7f21efc2842e - rustup::dist::component::package::unpack_without_first_dir::h99f4b0c2cae349ff
  17:     0x7f21efbe559f - rustup::dist::manifestation::Manifestation::update::hacf67fad3b44f03a
  18:     0x7f21efbd563c - rustup::dist::dist::update_from_dist_::h1462627f9c58c494
  19:     0x7f21efbd227a - rustup::install::InstallMethod::install::hbb6430b2dfaf5792
  20:     0x7f21efbd0d21 - rustup::toolchain::DistributableToolchain::install_from_dist::h7b9bf9317c9aa2c5
  21:     0x7f21efcc5aa2 - rustup::cli::self_update::install::h8ca0f61a773526a9
  22:     0x7f21efccbd3d - rustup::cli::setup_mode::main::h4d335e1871435d70
  23:     0x7f21ef8df42a - rustup_init::main::h22da971c634937a2
  24:     0x7f21efcf3e83 - std::rt::lang_start_internal::{{closure}}::{{closure}}::h4ed4ab1fb893cc93
                               at src/libstd/rt.rs:52
  25:     0x7f21efcf3e83 - std::sys_common::backtrace::__rust_begin_short_backtrace::h1f01c818c00c4f70
                               at src/libstd/sys_common/backtrace.rs:130
  26:     0x7f21ef8e053f - main
  27:     0x7f21ef4b70b3 - __libc_start_main
  28:     0x7f21ef8db029 - <unknown>
thread panicked while panicking. aborting.
Illegal instruction (core dumped)
```

環境


```:powershell
PS > wsl --list --verbose
  NAME                   STATE           VERSION
  Ubuntu                 Running         1
```

```:wsl
$ cat /etc/issue
Ubuntu 20.04.1 LTS \n \l
```

# 解決方法
## 1. WSL2にアップグレードする
WSL2ではインストールすることができました。
## 2. 古いバージョンのUbuntuを使う
Ubuntu18.04ではインストールすることができました。
## 3. 環境変数 RUSTUP_IO_THREADS=1 を設定する

```
export RUSTUP_IO_THREADS=1
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
```

環境変数 `RUSTUP_IO_THREADS=1` を設定するとWSL1, Ubuntu20.04 でもインストールすることができました！

参考: https://github.com/rust-lang/rustup/issues/2245