Prevent missing rust-src from reinstalling a nightly without miri

A build log from GitHub Actions:

    $ Run rustup toolchain install nightly --component miri --profile minimal --allow-downgrade --no-self-update
    info: syncing channel updates for 'nightly-x86_64-unknown-linux-gnu'
    info: latest update on 2021-10-27, rust version 1.58.0-nightly (e269e6bf4 2021-10-26)
    info: skipping nightly which is missing installed component 'miri'
    info: syncing channel updates for 'nightly-2021-10-26-x86_64-unknown-linux-gnu'
    info: latest update on 2021-10-26, rust version 1.58.0-nightly (29b124802 2021-10-25)
    info: downloading component 'cargo'
    info: downloading component 'miri'
    info: downloading component 'rust-std'
    info: downloading component 'rustc'
    info: installing component 'cargo'
    info: installing component 'miri'
    info: installing component 'rust-std'
    info: installing component 'rustc'

      nightly-x86_64-unknown-linux-gnu installed - rustc 1.58.0-nightly (29b124802 2021-10-25)

    $ Run rustup default nightly
    info: using existing install for 'nightly-x86_64-unknown-linux-gnu'
    info: default toolchain set to 'nightly-x86_64-unknown-linux-gnu'

      nightly-x86_64-unknown-linux-gnu unchanged - rustc 1.58.0-nightly (29b124802 2021-10-25)

    $ Run echo "::set-output name=version::$(rustc --version)"
    info: syncing channel updates for 'nightly-x86_64-unknown-linux-gnu'
    info: latest update on 2021-10-27, rust version 1.58.0-nightly (e269e6bf4 2021-10-26)
    warning: Force-skipping unavailable component 'miri-x86_64-unknown-linux-gnu'
    info: downloading component 'rust-src'
    info: downloading component 'cargo'
    info: downloading component 'rust-std'
    info: downloading component 'rustc'
    info: removing previous version of component 'cargo'
    info: removing previous version of component 'miri'
    info: removing previous version of component 'rust-std'
    info: removing previous version of component 'rustc'
    info: installing component 'rust-src'
    info: installing component 'cargo'
    info: installing component 'rust-std'
    info: installing component 'rustc'

    $ Run rustc --version
    rustc 1.58.0-nightly (e269e6bf4 2021-10-26)

    $ Run cargo miri test
    error: the 'miri' component which provides the command 'cargo-miri' is not available for the 'nightly-x86_64-unknown-linux-gnu' toolchain
    Error: Process completed with exit code 1.

.github/workflows/ci.yml

@@ -72,7 +72,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: dtolnay/rust-toolchain@nightly
         with:
-          components: miri
+          components: miri, rust-src
       - run: cargo miri test
 
   outdated: