Restart during an update install's wait can relaunch the old Desktop beside the installer #1654

Follow-up from #1625 (Known limits). Restart (settings app:relaunch or the picker) during an update install's wait still schedules app.relaunch(), so the old Desktop relaunches about 0.4 s after it exits and could collide with the real installer. Untested against the real update feed. A launch click (relaunchIfQuitting) already skips update installs.

Fix: skip app.relaunch() when the held quit is an update install, the same way relaunchIfQuitting does.