bug#71962: Flatpak applications fail to launch with systemd-related error

[Juliana Sims via Bug reports for GNU Guix <bug-guix@gnu.org>]

Hello,

I ran a system reconfigure for the first time in a while today. After 
doing so, I noticed that Flatpak apps fail to launch. Specifically, 
when running `flatpak run org.gnome.Fractal`, which was working fine 
before, the application hangs instead of launching and, with `-v`, 
prints the following:

```
F: No installations directory in 
/gnu/store/v4dy0sfj7znnzar1nn18jz0aznjni2fv-flatpak-1.14.6/etc/flatpak/installations.d. 
Skipping
F: Opening system flatpak installation at path /var/lib/flatpak
F: Opening user flatpak installation at path 
/home/user/.local/share/flatpak
F: Opening user flatpak installation at path 
/home/user/.local/share/flatpak
F: Opening user flatpak installation at path 
/home/user/.local/share/flatpak
F: 
/home/user/.local/share/flatpak/runtime/org.gnome.Platform/x86_64/46/1ad35c08f997c0257506b34db175a27438799f0e5dc0f5e56ebe3635f53cbf58/files/lib32 
does not exist
F: Cleaning up unused container id 181884517
F: Cleaning up per-app-ID state for org.gnome.Fractal
F: Allocated instance id 3966080007
F: Add defaults in dir /org/gnome/Fractal/
F: Add locks in dir /org/gnome/Fractal/
F: Allowing dri access
F: Allowing wayland access
F: Allowing pulseaudio access
F: Failed to run in transient scope: No systemd user session available, 
cgroups not available
F: Running 
'/gnu/store/y6l9z58xm8jzj7n1vxi5mcdb90x4sgal-bubblewrap-0.8.0/bin/bwrap 
--args 35 -- 
/gnu/store/kxlbkakfs2b7m30vyr4r1gkb4zwzfi9p-xdg-dbus-proxy-0.1.5/bin/xdg-dbus-proxy 
--args=37'
```

Of these messages, I suspect the culprit is "No systemd user session 
available."

I had elogind installed through the GNOME system service while flatpak 
was installed in my user home profile. I moved flatpak to the system 
profile and reconfigured the system; this did not help.

Best,
Juli