added caching to ci builds to reduce build times

[blake-mealey]

This adds caching to the CI workflow which should improve build times quite a bit. I used the recommended cache parameters from here: https://github.com/actions/cache/blob/main/examples.md#rust---cargo

I tried this approach in my project and it took build times from ~1min to ~10s:

• before: https://github.com/blake-mealey/rocat/runs/3999515438?check_suite_focus=true
• after: https://github.com/blake-mealey/rocat/runs/4015757268?check_suite_focus=true

Note that I haven't added caching to the release workflow. We could do this as well but I don't think it'll give you any benefit since whenever you run that workflow your Cargo.toml has changed (bumped version number) and therefore it will always have a cache miss.

[Kampfkarren]

IMHO--caches in CI are kind of spooky, CI times on GitHub Actions aren't bad enough to warrant them, and if they cache when they shouldn't you can get some nasty to debug problems.

[LPGhatguy]

This is neat, but @Kampfkarren's concerns about cache spookiness resonate with me. I don't think our CI is quite slow enough for me to want to take this on yet!

I'd like to keep this PR open for future reference.