GTK 4 applications use accessibility bus directly
and will try to connect to it every time a widget is created:
https://gitlab.gnome.org/GNOME/gtk/-/issues/4831
This will make GTK 4 apps (e.g. newly ported Nautilus) grind
to a halt on systems that do not have AT-SPI service installed.
Let’s explicitly disable accessibility support with an environment
variable when the AT-SPI service is not enabled to avoid that.
Just like we do for ATK-based applications with `NO_AT_BRIDGE`.
Fixes: https://github.com/NixOS/nixpkgs/issues/197188
2fedbbab9c