ci: pin netcdf-fortran@4.6.1 for macOS extended build tests

Author: wpbonelli

.github/actions/test-extended/action.yml

@@ -19,7 +19,7 @@ runs:
       if: runner.os == 'macOS'
       shell: bash
       run: |
-        brew install netcdf-fortran libevent
+        brew install netcdf-fortran@4.6.1 libevent
         nc-config --all
         nf-config --all