This adds:
 - pub's cache to the Cirrus cache. It caches based on the checksums in the pubspec (although pub has things in versioned folders, this'll prevent the cache just getting bigger over time).
 - bin/cache/pkg. I think this only includes sky_engine. It caches based on the engine checksum.
 - bin/cache/artifacts. This includes engine, gradle_wrapper, material_fonts. Also caches based on the engine checksum.