bug#26202: Guix in foreign distributions: setting XDG_DATA_DIRS causes user session to not start

[Adonay Felipe Nogueira]

* Steps to reproduce

1. Edit "~/.profile" and add:

export GUIX_PROFILE="${HOME}/.guix-profile"
export XDG_DATA_DIRS="${GUIX_PROFILE}/share${XDG_DATA_DIRS:+:}$XDG_DATA_DIRS"

2. Log out and try to log in again.

* Current behavior

The *host's* GNOME Flashback session (gnome-session
--session=gnome-flashback) attempts to start, however, it's aborted
shortly after successful log-in.

"~/.xsession-errors" file shows something similar to:

# Begin of file
gnome-session: GLib-GIO-ERROR: Settings schema
'org.gnome.SessionManager' is not installed.

aborting...
# End of file

* Additional information

- In this case, I'm using Trisquel 7.
- `gnome-session` command is provided by the
  "gnome-session-bin", version 3.9.90-0ubuntu12.1+7.0trisquel2.
- Display manager is lightdm (Trisquel 7's default), version
  1.10.6-0ubuntu1+7.0trisquel1.
- Being a "~/.profile" setting, the problem is not found unless the user
- logs-out and back in.

* Personal comment

This might become a problem if Guix starts suggesting people to
customize $XDG_DATA_DIRS in their "~/.profile".


Respectfully, Adonay.
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