override: Fix completion for --system/--user
override: Fix completion for --system/--user

These options were not showing up in completion,
despite being accepted by the override command.

Closes: #2534
Approved by: alexlarsson

matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak