This PR adjusts the way we cache Cargo dependencies in CI. We're trying out [swatinem/rust-cache](https://github.com/swatinem/rust-cache) to see if it can improve our caching strategy such that we're able to get more cache hits on PRs. We'll only write to the cache on `main` in the hopes that it will mitigate the amount of thrashing of the cache. Release Notes: - N/A