CI: Use system Python 2.7 on macOS instead of trying to install a separate one with "setup-python", which fails.

Author: scoder

.github/workflows/ci.yml

@@ -124,6 +124,7 @@ jobs:
 
     - name: Set up Python
       uses: actions/setup-python@v4
+      if: startsWith(matrix.python-version, '3.')
       with:
         python-version: ${{ matrix.python-version }}