d/gnome-session_gnomerc.sh: Make GNOME and GNOME-derived have the same behavior
The extra double quotes around $XDG_CURRENT_DESKTOP make the Internal Field Separator (IFS) ':' fail to split the string held by the environment varialbe $XDG_CURRENT_DESKTOP. So, GNOME-derived will returns earlier and /usr/share/gnome will not be listed in the environment variable $XDG_DATA_DIRS.
This commit fixes the issue to make GNOME-derived have the same behavior as GNOME and the same environment variable $XDG_DATA_DIRS.
Fixes: commit d8cbb69e ("d/gnome-session_gnomerc.sh: Quote more defensively")