Comment # 3 on bug 1185647 from Dario Faggioli

Neal suggested trying:

 pkcon refresh force # Refresh the list of packages.
 pkcon --only-download update # Download the packages to update. Exit code is 5
 if there's no update.
 pkcon offline-trigger # Make sure system will boot into "offline upgrade"
mode.
 systemctl reboot # reboot

I've done it, and I see the same error that I was seeing in GNOME Software (in
that case, when clicking on 'Restart & Update'):

dario@localhost:~> pkcon offline-trigger
Command failed:
GDBus.Error:org.gtk.GDBus.UnmappedGError.Quark._pk_2dengine_2derror_2dquark.Code1:
Failed to create symlink: Read-only file system

And again, the reboot does not trigger the update.