Fix xdg-desktop-portal-gtk spam in greeter session

The greeter's niri session triggered D-Bus activation of
xdg-desktop-portal-gtk, which failed 17x on every boot because
WAYLAND_DISPLAY is not set in the greeter's D-Bus scope.
Setting GTK_USE_PORTAL=0 prevents GTK from requesting portal
services the greeter does not need.

defaults/etc/greetd/niri-greeter.kdl

@@ -35,6 +35,12 @@ layout {
     }
 }
 
+// Greeter needs no portal services — prevent GTK from triggering
+// xdg-desktop-portal activation (which fails without a proper display).
+environment {
+    "GTK_USE_PORTAL" "0"
+}
+
 // Start moongreet and quit niri once moongreet exits.
 // Retry loop ensures niri shuts down even on early moongreet crashes.
 spawn-sh-at-startup "moongreet; while ! niri msg action quit --skip-confirmation 2>/dev/null; do sleep 0.5; done"