FlatpakBuilder: Add xa.metadata as ostree commit metadata for runtimes
    
    Newer versions of flatpak (after https://github.com/flatpak/flatpak/commit/116a8b848788c582fb2c8577f1250438b100da25)
    verify that the metadata matches ostree commit xa.metadata key. As we
    use ostree directly for creating runtimes, we need to add the key
    manually here.
    
    Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1822753