ci: Add condition to sign GitHub release only for tagged releases

Author: ctoth

.github/workflows/publish-to-pypi.yml

@@ -88,6 +88,7 @@ jobs:
 
   github-release:
     name: Sign the Python 🐍 distribution 📦 with Sigstore and upload to GitHub Release
+    if: startsWith(github.ref, 'refs/tags/')  # Only run for tagged releases
     needs:
     - publish-to-pypi
     runs-on: ubuntu-latest