Install GCC 9 on MacOS if not available

Author: awvwgk

.github/workflows/CI.yml

@@ -48,6 +48,7 @@ jobs:
     - name: Install GFortran macOS
       if: contains(matrix.os, 'macos')
       run: |
+        brew install gcc@${GCC_V}
         ln -s /usr/local/bin/gfortran-${GCC_V} /usr/local/bin/gfortran
         which gfortran-${GCC_V}
         which gfortran