In Gitlab CI, since gcc 7 crashes when building mfem, try to use newer gcc

for building mfem

share/spack/gitlab/cloud_pipelines/stacks/radiuss/spack.yaml

@@ -9,6 +9,11 @@ spack:
       providers:
         mpi: [mvapich2]
 
+    mfem:
+      require:
+      # gcc@7.5.0 crashes when building mfem@4.8.0
+      - '@gcc@12'
+
   specs:
   - ascent  # ^conduit@0.6.0
   - axom