Don't compress the output tarball
    
    There's very minimal gains from recompressing the output tarball, since
    the layers are already compressed, and a lot of time is spent done doing that
    when the layers are big; just write an uncompressed tarball.
    
    (If we want to save space, the right way to do it is to switch the
    *layers* to zstd -9, saving time when downloading layers in the
    registry, not just in intermediate storage.)
    
    This conflicts with older versions of Koji flatpak that expect the
    .tar.gz output.