Updated the GitHub Actions script to include caching

[Kixiron]

• Added caching to the Actions flow
  • Caches the cargo registry at ~/.cargo/registry
  • Caches cargo binaries at ~/.cargo/bin
  • Caches cargo git at ~/.cargo/git
  • Caches /target build artifacts
  • Caches based off of the hash of the Cargo.lock, so changes that alter it will require full rebuilds
  • Will hopefully provide faster CI build/test times

[jyn514]

r? @pietroalbini

[pietroalbini]

I don't think this is effective in the long term: GitHub Actions's cache is immutable, and the same key won't be reuploaded if the files on disk change. Files in target/ are regenerated after rustc updates, so we'll need to include the rustc version in the cache key.

You can see how crates.io does it.

[Kixiron]

Anything that speeds up builds is a plus in my books, do you prefer cargo's caching for build artifacts + caching cargo artifacts?

[pietroalbini]

I think this PR is fine if the rustc version is added to the cache keys. Otherwise once a new stable version is released we'd lose all the performance boost of caches.

[pietroalbini]

Great, thanks!