gdk: Replace GTK_USE_PORTAL env var with GDK_DEBUG flag
It's a debug flag, so make it clear that it is one.
Related: Clowns on the Arch wiki on https://wiki.archlinux.org/title/Uniform_look_for_Qt_and_GTK_applications#Consistent_file_dialog
It's a debug flag, so make it clear that it is one.
Related: Clowns on the Arch wiki on https://wiki.archlinux.org/title/Uniform_look_for_Qt_and_GTK_applications#Consistent_file_dialog