Switch from actions-rs to preinstalled rustup

[tgross35]

actions-rs is deprecated. Switch to using the preinstalled rustup to install the toolchain, and https://github.com/Swatinem/rust-cache to configure cacheing.

https://github.com/dtolnay/rust-toolchain cannot be used because it does not respect rust-toolchain.

I believe there isn't any need to run cargo build separately anymore since there are no longer error annotations.

Fixes #381

[tgross35]

@antoyo does matrix items libgccjit_version or commands (link) affect what is built? If so, maybe they should be part of the cache key

[antoyo]

@antoyo does matrix items libgccjit_version or commands (link) affect what is built? If so, maybe they should be part of the cache key

No, rustc_codegen_gcc itself is built with LLVM, so the crates used by it are not affected by those flags.

[antoyo]

It seems like you forgot to remove actions-rs in the gcc12 CI.

[tgross35]

Good catch, removed that

[antoyo]

Thanks a lot!