Disable zchunk metadata creation for now
    
    Rawhide and Branched createImage tasks are frequently failing
    with "error: Curl error (16): Error in the HTTP2 framing layer"
    when trying to download the zchunk metadata. We have not yet
    been able to work out why this is, but it is a serious problem
    causing many composes to fail and badly impeding our testing
    of Rawhide and F30. For this reason we've decided to disable
    zchunk metadata generation until this is fixed.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>