bug#36508: GDM files have incorrect owner after temporarily replacing with SDDM

[ison <ison@airmail.cc>]

After replacing GDM with SDDM in my Guix System config (to test Wayland) and
then reverting back to my old config and reconfiguring GDM would crash
(printing out around 500 lines about creating a seat)
I also tried rolling back to the generation I had before using SDDM and it would
still crash.
In both instances I also tried "herd restart xorg-server" but same problem.

I then checked the log file /var/log/gdm/greeter.log which had errors such as:
-------------------
Fatal server error:
(EE) Cannot open log file "/var/lib/gdm/.local/share/xorg/Xorg.pid-720.log"
-------------------

And then I could verify that files inside of /var/lib/gdm had incorrect
ownership of 9##:gdm
where 9## was some 3-digit number I can't remember now.
(note: the directory itself /var/lib/gdm still had correct ownership gdm:gdm)

I then manually fixed the ownership with:
chown -R gdm:gdm /var/lib/gdm
and GDM successfully came up without crashing.

The relevant portion of my config when I replaced GDM with SDDM was:
-------------------------------
(operating-system
  ...
  (services
    (cons*
      ...
      (sddm-service
        (sddm-configuration
          (display-server "wayland")))

      ;; Return %desktop-services with GDM removed
      (remove (lambda (service)
                (eq? (service-kind service) gdm-service-type))
              %desktop-services))))
-------------------------------