Switch to Swatinem/rust-cache

[BD103]

This is an experiment to see if this cache action performs better than Leafwing-Studios/cargo-cache, just like TheBevyFlock/bevy_new_2d#381. If this gets merged we can test it for a few weeks then evaluate whether it's an improvement or not.

Closes #400.

[BD103]

There are some benefits to this change:

1. Caches will only be saved on the main branch. This prevents our cache storage limit from being filled up too quickly when there are multiple active PRs.
2. Action only caches dependencies, not the actual crates in the workspace. (This means things like bevy, toml, and serde will be cached, but not bevy_lint or bevy_cli.)
3. Incremental compilation is disabled, since it's only used to rebuild workspace crates like bevy_cli and bevy_lint, takes up a lot of disk space, and can grow outdated quickly.
4. Implements a better system than cargo-sweep that clears artifacts that have been unmodified for over a week. (cargo-sweep uses access time, but many OS's don't update access time properly for performance reasons.)

I'm curious to see how well this avoids rebuilding dependencies and reduces our cache sizes.

[DaAlbrecht]

I was pretty happy with our CI, the build times were really good in the end but excited to try this out!

[BD103]

So far it seems like the overall runtime has decreased from around 14 minutes to 6 minutes. We'll check back in a few weeks to see if runtimes manage to stay that fast! We can use the actions performance metrics for this. I also deleted all current caches, so we're about to start fresh.