feat, ci: skip existing distros on test pypi

This helps us avoid a race condition when a tag is pushed before the
`main` build is done.

.github/workflows/ci.yaml

@@ -59,6 +59,7 @@ jobs:
           user: __token__
           password: ${{ secrets.TEST_PYPI_API_TOKEN }}
           repository_url: https://test.pypi.org/legacy/
+          skip_existing: true
       - name: Publish to PyPI
         if: github.event_name == 'push' && startsWith(github.ref, 'refs/tags')
         uses: pypa/gh-action-pypi-publish@release/v1