Take mgard out of clang7_nopy_ompi_h5_ad2_libcpp run

Author: franzpoeschel

.github/ci/spack-envs/clang7_nopy_ompi_h5_ad2_libcpp/spack.yaml

@@ -12,7 +12,7 @@ spack:
 
   packages:
     adios2:
-      variants: ~zfp ~sz ~png ~dataman ~python ~fortran ~ssc ~shared ~bzip2
+      variants: ~zfp ~sz ~png ~dataman ~python ~fortran ~ssc ~shared ~bzip2 ~mgard
     cmake:
       externals:
       - spec: cmake@3.23.0