Update Rust action in CI and simplify caching

[TimJentzsch]

Closes #124.

In this PR, the mostly unmaintained actions-rs/toolchain action is replaced by dtolnay/rust-toolchain which is also used in Bevy's CI now.

Additionally, the Leafwing-Studios/cargo-cache action is used to greatly simplify the build file caching in the workflows.

[janhohenheim]

I was eyeing the dtolnay action as well, but I'm a bit sad that it doesn't support the toolchain file at the root. Then again, the toolchain file would need to include both macOS targets to be useful in the CI, but we don't want that for non-CI builds...
Anyways, that's just rambling, thanks for the PR! I'll merge it when the CI checks pass :)
Maybe @NiklasEi would be glad to have this upstreamed in https://github.com/NiklasEi/bevy_game_template as well. What do you think, Niklas?

[TimJentzsch]

I'm a bit sad that it doesn't support the toolchain file at the root.

Doesn't the action just install Rust and Cargo?
In theory, the cargo commands should still use the toolchain file if I'm not mistaken.
Or do you mean that it should automatically install the correct version of rustc by parsing the toolchain file?

[janhohenheim]

Exactly. Currently, because of madsmtm/objc2#432, there is a specific nightly version duplicated in both the toolchain file and the workflow.

[TimJentzsch]

Exactly. Currently, because of madsmtm/objc2#432, there is a specific nightly version duplicated in both the toolchain file and the workflow.

Have you tried this patch already with cargo update? madsmtm/objc2#432 (comment)

[janhohenheim]

Not yet, been busy with the Jam ;D