proxy/gnome: check GNOME_DESKTOP_SESSION_ID too
proxy/gnome: check GNOME_DESKTOP_SESSION_ID too

It claims to be deprecated, but $GNOME_DESKTOP_SESSION_ID seems to be
the only reliable identifier of GNOME-ness (other than probing D-Bus,
which we can't do from here).

https://bugzilla.gnome.org/show_bug.cgi?id=701377

GNOME/glib-networking,GNOME/glib-networking,GNOME/glib-networking