Try using env.ImageOS as part of cache key

Author: danielhollas

.github/workflows/gfortran.yml

@@ -75,7 +75,7 @@ jobs:
       with:
         path: ~/pfunit/build/installed
         # To force a pFUnit rebuild (bust the cache), make a change to install_pfunit.sh
-        key: ${{ runner.os }}-ubuntu22.04-pfunit-gfortran${{ env.GCC_V }}-${{ hashFiles('dev_scripts/install_pfunit.sh') }}
+        key: ${{ runner.os }}-${{ env.ImageOS }}-pfunit-gfortran${{ env.GCC_V }}-${{ hashFiles('dev_scripts/install_pfunit.sh') }}
 
     - name: Download and build pFUnit
       if: steps.pfunit-cache.outputs.cache-hit != 'true'