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.