ci: add --fresh flag to gcc installation

Force fresh concretization to avoid pulling gcc-runtime from public
buildcache which may be built with an incompatible gcc version.

Author: dennisklein

.github/workflows/buildcache.yml

@@ -52,7 +52,7 @@ jobs:
       - name: Install GCC
         shell: spack-bash {0}
         run: |
-          spack install gcc@${{ matrix.gcc }}
+          spack install --fresh gcc@${{ matrix.gcc }}
           spack compiler find $(spack location -i gcc@${{ matrix.gcc }})
 
       - name: Install dependencies