atspi: Fix sandbox check

We only want to check if we are running in a sandbox here, so
use the proper api for that.

gtk/a11y/gtkatspiroot.c

@@ -769,7 +769,7 @@ on_registration_reply (GObject      *gobject,
    * sandbox to allow event registration signals to propagate, so we
    * check if the version of the Flatpak portal is recent enough.
    */
-  if (gdk_should_use_portal () &&
+  if (gdk_running_in_sandbox () &&
       !check_flatpak_portal_version (7))
     {
       GTK_DEBUG (A11Y, "Sandboxed does not allow event listener registration");