Cache cargo dependencies in CI

[exdx]

It's possible to optimize the Rust CI by caching the cargo home directory, or specifically certain files under .cargo, and then sharing those files between different CI jobs. This avoids the need to re-download dependencies during multiple jobs.

See https://doc.rust-lang.org/cargo/guide/cargo-home.html#caching-the-cargo-home-in-ci for more details.

This could potentially be done for all of our Rust projects. Not sure which have this implemented already.