Add option to specify rust-toolchain.toml path

[Svampen]

Our rust code doesn't live in the root folder because the project contains both JS and Rust code, making it impossible for the action to pick up our rust-toolchain.toml file.
It would be good if one could specify the path to the rust-toolchain.toml file(s) and have that be used.

We had a issue today with rust 1.82.0 in regards to Fastly that made us having to pin 1.81.0 but we had to do it in our rust-toolchain.toml and in the github action. This proposed option would make it less likely that we would forget to unpin the version in both places.

[jonasbb]

That sounds reasonable. Instead of a separate value, I think this could piggyback of the cache-workspaces input. That input allows specifying multiple workspaces, which might not work too well with the current installation code.

[Arignir]

This would be pretty cool indeed!

[hongbo-miao]

My mono repo has 11 Rust projects inside currently.
It would be great to support custom rust-toolchain.toml path such as

      - name: Set up Rust
        uses: actions-rust-lang/setup-rust-toolchain@v1.11.0
        with:
          rust-toolchain-file: path/to/rust-toolchain.toml

Thanks! ☺️