stage: avoid using group name in _first_accessible_path()
stage: avoid using group name in _first_accessible_path()

On machines where $TMP is owned by a gid with no name, this avoids the
following error when the default spack stage does not exist:

    (spackbook):spack$ spack clean
    ==> Removing all temporary build stages
    ==> Error: 'getgrgid(): gid not found: 57095'

Spack needs to deal with gids directly unless users pass them in.

iulian787/spack,iulian787/spack,LLNL/spack,iulian787/spack,LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,LLNL/spack