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.

.github/workflows/release.yml

@@ -96,6 +96,8 @@ jobs:
         with:
           submodules: recursive
       - uses: swatinem/rust-cache@v2
+        with:
+          save-if: ${{ github.ref == 'refs/heads/main' }}
       - name: Install cargo-dist
         run: ${{ matrix.install_dist }}
       - name: Build artifacts