nixos/river: fix display manager error

nixos/modules/programs/wayland/river.nix

···
         environment.systemPackages = optional (cfg.package != null) cfg.package ++ cfg.extraPackages;
 
         # To make a river session available if a display manager like SDDM is enabled:
-        programs.xwayland.enable = mkDefault true;
+        services.xserver.displayManager.sessionPackages = optionals (cfg.package != null) [ cfg.package ];
       }
       (import ./wayland-session.nix { inherit lib pkgs; })
     ]);