gh-85644: webbrowser: Use $XDG_CURRENT_DESKTOP to check desktop (GH-21731)

Usage of $GNOME_DESKTOP_SESSION_ID env variable is deprecated since
GNOME 3.30.0 [1], so should not be used, while the standard
XDG_CURRENT_DESKTOP should be instead preferred.

[1] https://gitlab.gnome.org/GNOME/gnome-session/-/commit/00e0e6226371d53f65