Don't offer Fedora distro upgrades if we don't have correct artwork

Make sure we have the artwork for the distro upgrade banner before
offering an upgrade. In practice, this means that we don't offer a
distro upgrade at all unless the user has first updated gnome-software
to the version available from in the updates repo.

This comes with a slight tradeoff that some people who never install
regular updates will not get the distro upgrade notification at all
after this change.
Status Job ID Name Coverage
  Build
passed #700474
debian-stable-x86_64

00:01:22

passed #700473
fedora-x86_64

00:03:22