ostree-installer: download Flatpaks from registry-no-cdn.fedoraproject.org
    
    The koji runroot used for to create the OSTree installer has no access
    to external networks, so use a special hostname for registry.fedoraproject.org
    that suppresses the redirects to the CDN that would normally happen for
    blob downloads.
    
    Signed-off-by: Owen W. Taylor <otaylor@fishsoup.net>