openqa/worker: install ffmpeg-free
    
    I'm adding this as a Recommends: for os-autoinst, but want to
    get it on the workers now. Having it installed gives us better
    videos of test runs (the internal video encoder is a bit wonky
    and produces videos that have errors which make jumping around
    within the video not work properly).
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>