rpm: restore pipewire units on pulseaudio-qubes removal

Since pipewire units are getting disabled on pulseaudio-qubes install,
reverse the action when pulseaudio-qubes is getting removed (and usually
replaced with pipewire-qubes). Note the %postun is called on the upgrade
too, so use 'preset' (which will DTRT once 75-pulseaudio-qubes.preset is
gone) instead of 'disable' directly.

QubesOS/qubes-issues#6358

rpm_spec/gui-agent.spec.in

@@ -187,6 +187,14 @@ else
     preset_units /usr/lib/systemd/user-preset/75-pulseaudio-qubes.preset upgrade
 fi
 
+%postun -n pulseaudio-qubes
+
+for unit_name in pipewire.socket pipewire.service \
+                 pipewire-pulse.socket pipewire-pulse.service \
+                 wireplumber.service; do
+    systemctl --no-reload --global preset "$unit_name" >/dev/null 2>&1 || :
+done
+
 %post -n pipewire-qubes
 %systemd_user_post pipewire.service pipewire.socket wireplumber.service