There is a possiblilty of the apt-get post hook getting triggered
more than once for each apt-get session, therefore we only notify
dom0 that there are no updates available and do not perform an
apt-get update.
The qubes-update-check.service will still perform an update so even
if the dist-upgrade failed and there was actually more files to update
the qubes-update-check.serivce would then at some point notify dom0
about those updates being available
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