Ability to install without setting override

[jayvdb]

This action has one significant difference to https://github.com/actions-rs/toolchain in that it defaults to overriding the toolchain, whereas the old action only does that if override: true is set.

It would be nice to be able to opt out of that override.

[jonasbb]

The override is necessary as otherwise it is not possible to use a different toolchain than specified in the rust-toolchain.toml see #26 and #31. Can you please explain what your use case is and why the current way is a problem for you?

The override isn't anything special, so you can get rid of it by executing rustup override unset after this action. I hope that helps.

[jayvdb]

We install nightly only for its rustfmt , and dont want to build with it.

Yes, I am using rustup override after this github action in order to clear it. I would prefer not having the extra step, by telling this action that I do not want it.

[jonasbb]

I added this as an option in 4d1965c9142484e48d40c19de54b5cba84953a06.