bug#39281: gdm doesn't start

[Jack Hill]

I believe that I've found the problem!

Using the technique that Ludo’ described in 
<https://issues.guix.gnu.org/issue/34580#17>, I was able to grab an strace 
of gdm (attached) while it was failing to start properly. I was suspicious 
of the following line:

"""
418   22:46:10 sendto(8, "<13>Jan 26 22:46:10 gdm: GdmManager: couldn't 
look up username gnome-initial-setup\n", 83, MSG_NOSIGNAL, NULL, 0) = 83 
<5.487224>
"""

so I removed gnome-initial-setup from the propagated-inputs with the 
following patch, and, indeed, that resolved the problem with gdm on my 
system.

While I'm still not exactly sure why this was causing gdm problems, and 
why `herd restart xorg-server` caused gdm to start working correctly, I 
think that removing gnome-initial-setup from the gnome meta-package is the 
right thing to do. Instead, perhaps it could be provided via its own 
service, auto-selected by the installer. If I recall correctly from other 
distros (e.g. Fedora), the gnome-initial-setup wizard is run from its own 
user on first boot, and after it finishes, the user is logged in as 
themselves. That said, gnome-initial-setup did seem to run fine for me the 
first time I logged into gnome after it was installed, and hasn't stared 
again.

Thoughts?
Jack

P.S. Thanks Raghav for your work on making gnome in guix a better 
experience!

gdm.log
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