workstation: Include cached metadata for PackageKit
    
    Instead of taking the metadata from PackageKit-cached-metadata package
    as we were doing previously, copy it over directly during the compose
    from https://kojipkgs.fedoraproject.org/mash/
    
    This makes it much less error prone as we always get the very latest
    metadata, and makes maintenance much simpler as we don't need to roll
    PackageKit-cached-metadata by hand. Users are also going to appreciate
    this because it makes post-GA updates smaller as they won't have to
    download updates for the PackageKit-cached-metadata subpackage each time
    PackageKit gets updated.