Ensure repos are set up for cockpit tests
    
    When running these tests on updates we inherit the main disk
    image from server_cockpit_default, which has the repo config,
    but the actual repo data is on HDD_3 which is not inherited.
    We need to re-download the updates here to ensure they're
    available to the tests (the automatic update test installs the
    dnf-automatic package, so it should pull it from the update repo
    if it is part of the update being tested).
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>