Fix: MacOS by pinning to MacOS-11 runner.

Author: LKedward

.github/workflows/CI.yml

@@ -21,14 +21,14 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest]
+        os: [ubuntu-latest, macos-11, windows-latest]
         gcc_v: [9] # Version of GFortran we want to use.
         include:
         - os: ubuntu-latest
           os-arch: linux-x86_64
           release-flags: --flag '--static -g -fbacktrace -O3'
 
-        - os: macos-latest
+        - os: macos-11
           os-arch: macos-x86_64
           release-flags: --flag '-g -fbacktrace -O3'
 
@@ -48,7 +48,6 @@ 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