tighten conditions under which pypi upload is attempted

Author: jepler

.github/workflows/build.yml

@@ -127,7 +127,7 @@ jobs:
         [ -z "$AWS_ACCESS_KEY_ID" ] || aws s3 cp circuitpython-stubs/dist/*.tar.gz s3://adafruit-circuit-python/bin/stubs/circuitpython-stubs-${{ env.CP_VERSION }}.zip --no-progress --region us-east-1
 
     - name: Upload stubs to PyPi
-      if: github.event_name == 'push'
+      if: github.event_name == 'push' && github.ref == 'refs/heads/main' && github.repository_owner == 'adafruit'
       env:
         TWINE_USERNAME: ${{ secrets.pypi_username }}
         TWINE_PASSWORD: ${{ secrets.pypi_password }}