Do not load apps when an upgrade is due

This makes it still possible to update from the command line, but
disables custom commands from apps

This fixes app updates happening too early when triggering a CLI update.

Please review @icewind1991 @DeepDiver1975 @bantu 

Note: you can simulate an app update.php fail by increasing the app's version number and adding `throw new \Exception("simulated exception");` in the app's `update.php` file.