Consider caching CI

[mattxwang]

Jotting this down so I don't forget - we should try to cache our cargo dependencies so that our CI build times are shorter.

Coupe thoughts:

• might be a good idea to commit Cargo.lock
• seems like there's no first-party GitHub Action for this - https://github.com/Swatinem/rust-cache seems like a good start
• this blog post seems to indicate that there are some other microoptimizations we could look at, most notably https://github.com/nextest-rs/nextest - but I think we can table that until later