Fix github reference for publishing action (#1159)

Author: bcantoni

.github/workflows/publish-pypi.yml

@@ -34,7 +34,7 @@ jobs:
           repository_url: https://test.pypi.org/legacy/
 
       - name: Publish distribution 📦 to PyPI
-        if: ${GITHUB_REF} == 'refs/heads/master'
+        if: ${{ github.ref == 'refs/heads/master' }}
         uses: pypa/gh-action-pypi-publish@release/v1  # license BSD-2
         with:
           password: ${{ secrets.PYPI_API_TOKEN }}