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>
