Use unstable publish action (#118)

Our most recent publish step failed:
https://github.com/foxglove/foxglove-python/actions/runs/13660769163/job/38191137604

We appear to need this fix:
pypa/gh-action-pypi-publish#340 ...which is in
[unstable/v1](https://github.com/pypa/gh-action-pypi-publish/commits/unstable/v1)
but not yet in a versioned release.

Author: bryfox

.github/workflows/ci.yml

@@ -41,6 +41,6 @@ jobs:
         if: |
           !github.event.pull_request.head.repo.fork &&
           startsWith(github.ref, 'refs/tags/releases/v')
-        uses: pypa/gh-action-pypi-publish@release/v1
+        uses: pypa/gh-action-pypi-publish@unstable/v1
         with:
           packages_dir: dist