Only write to rust cache in CI from main branch (#874)
Each cache entry is ~1 GB of our allotted 10 GB for the repository which
is quite a bit. We're probably losing cache entries all the time since
we add an entry per commit per pull request.
Saving the cache takes ~3 minutes
([example](https://github.com/astral-sh/puffin/actions/runs/7479909295/job/20358124969)),
it's probably just slowing down CI. It's ~25% of our test runtime and
~50% of our clippy runtime.
