nixos/orca: Make available to gdm greeter

Without this, the greeter is unable to see `orca` program on `PATH`
and so `${gdm}/share/gdm/greeter/autostart/orca-autostart.desktop`
will fail to start it. As a result, screen reader would not work
on the login screen.

nixos/modules/services/accessibility/orca.nix

···
 
   config = mkIf cfg.enable {
     environment.systemPackages = [ cfg.package ];
+    systemd.services.display-manager.path = [ cfg.package ];
     services.speechd.enable = true;
   };
 }