Add functionality to use MSRV from rust-version field

[KamilaBorowska]

It would be useful if rust-toolchain could use whatever version is provided in rust-version field in Cargo.toml. For example, it could look like this:

- uses: dtolnay/rust-toolchain@msrv
  working-directory: project-within-workspace

[dtolnay]

I would prefer not to support this.

[alerque]

Can we ask why or for a re-consider? Obviously this might not be the default, but having some option to use the MSRV toolchain specified in the Cargo would add some consistency to CI runs that specifically test whether the named MSRV is working. Having to maintain the same version in two places to test it is not ideal. For example I just saw this come up here:

typst/typst#1402 (comment)

[tamird]

A reasonable workaround:

      - name: Read crate metadata
        id: metadata
        run: echo "rust-version=$(sed -ne 's/rust-version *= *\"\(.*\)\"/\1/p' Cargo.toml)" >> $GITHUB_OUTPUT

      - name: Install Rust
        uses: dtolnay/rust-toolchain@master
        with:
          toolchain: ${{ steps.metadata.outputs.rust-version }}

[nyurik]

I agree that this is seems like a needed feature - the action supports more exotic things like stable 18 months ago, but not a simpler feature like MSRV?

@dtolnay are there any specific reason you are thinking of that I don't see? Thanks for all the hard work! :)