PackageKit is used only by GUI updaters, refresh its cache only then.
Since PackageKit daemon do not read dnf.conf, it doesn't know the right
repository location, so try to access network, which results in
timeouts. But at the same time, it invalidate previous cache, which is
what we need.
Do not delay every qubes-dom0-update call by pkcon call.
Apparently when package is signed with DSA key, rpm -K output is totally
different. This is the case for bumblebee package on rpmfusion.
Fixesqubesos/qubes-issues#988