image-build/osbuild: Pull ISOs into the compose
    
    OSBuild tasks can produce ISO files. If they do, we should include them
    in the compose, and we should pull them into the iso/ subdirectory
    together with other ISOs.
    
    Fixes: https://pagure.io/pungi/issue/1657
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>