Support pinning the rustup version

[Techassi]

This PR enables users to pin the rustup version used in this action, it defaults (to be non-breaking) to the current latest version (1.28.2). This would need updating in the future as soon as new rustup versions are available.

Note: I'm not one hundred percent certain that setting the env var on Windows works the same as on Linux-based systems. I have the suspicion we might need to use the set command on Windows. Please feel free to suggest how this is best tested. EDIT: The CI runs seem to suggest setting the env var worked as expected.

[dtolnay]

This would need updating in the future as soon as new rustup versions are available.

I am not interested in needing to do this. It would need to use rustup's latest release by default.

[Techassi]

Alright, makes sense. I will update the PR accordingly asap.

[dtolnay]

What is the intended behavior of

- uses: dtolnay/rust-toolchain
  with:
    rustup-version: N

when the runner contains a preinstalled rustup that is different from N?

[Techassi]

That's a tough question.

One option could be to add a force flag input, which forces re-installation resulting in the following behaviour:

• If installed in a different version: install the requested version
• If installed in the same version: don't download, just use the installed version

[dtolnay]

I think it would be better if this were handled by a separate action specifically for installing rustup. This action will just use whatever rustup has been installed by the time that this action runs.