CI: Cache build artifacts (maybe)

[Stebalien]

Right now we only cache the proof parameters but we don't cache the rust build artifacts. Rebuilding everything from scratch every time takes 5m per CI run. We should try to find a way to cache these artifacts.

On the other hand, I've run into other issues when caching rust build artifacts (namely, the cache filling up) so I'm not 100% sure this is worth it.