message: Fix use spack_branch instead of spack_commit
diff: "package_final_phase" : "configure",
  "package_source_dir" : "../..",
  "spack_url": "https://github.com/spack/spack",
  -"spack_commit": "mfem-updates",
  +"spack_branch": "mfem-updates",
  "spack_activate" : {}
  }
  