GNOME 42-rc notifies of available updates when `org.gnome.software.download-updates` is set false

This is an upstream report of downstream issue https://bugzilla.redhat.com/show_bug.cgi?id=2064077 . In Fedora live images we try to disable background downloading of updates and notification of their availability - which is obviously undesirable in a live environment - by setting org.gnome.software.download-updates to false. This has worked for a long time, but since gnome-software 42-rc appeared in Rawhide it no longer works, we see a notification that updates have been downloaded and are ready to be installed when booting live. This wastes RAM (which is the only place we can put downloaded data in a live environment), and the updates cannot possibly be installed.

Some of the recent changes to src/gs-update-monitor.c look kinda suspicious here, but I don't entirely understand how everything in there works, so I won't point fingers any more specifically than that for now :D