Make debian call post-install hook after update

[ben-grande]

Other distributions already did it:

• DNF calls it in qubes.post-update.actons
• Pacman calls it in qubes-update-desktop.icons.hook

Without running qubes.PostInstall, a system might not report all of its features.

Fixes: QubesOS/qubes-issues#10532
For: QubesOS/qubes-issues#1512

[marmarek]

First, this will call it after apt-get update, which only refreshes metadata, not update any packages.

But then, see also QubesOS/qubes-issues#10456.
And finally, the 4.2 -> 4.3 upgrade surely does change files in /etc/qubes-rpc, which does trigger the /etc/qubes-rpc/qubes.PostInstall (see debian/qubes-core-agent.triggers and matching .postinst). If calling it during upgrade didn't advertise new features, calling it again during the same upgrade likely wouldn't help either. This needs finding out why it didn't work in the first place.

FWIW I just tried 4.2->4.3 upgrade of a debian-12 template (with qubes-dist-upgrade --releasever=4.3 --template-standalone-upgrade) and it did announced new features...

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.89%. Comparing base (f0089ce) to head (906c876).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #628      +/-   ##
==========================================
- Coverage   70.30%   69.89%   -0.41%     
==========================================
  Files           3        3              
  Lines         495      495              
==========================================
- Hits          348      346       -2     
- Misses        147      149       +2     

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