nixos/display-managers: Use dbus socket unit shipped by upstream

This ensures a correct DBUS_SESSION_BUS_ADDRESS environment variable
is set and imported into the systemd user environment.

Previously this would refer to a non-existing path preventing commands
interacting with the systemd manager from working.

Closes NixOS#87502

nixos/modules/services/x11/display-managers/default.nix

@@ -39,7 +39,7 @@ let
 
       ${optionalString cfg.startDbusSession ''
         if test -z "$DBUS_SESSION_BUS_ADDRESS"; then
-          exec ${pkgs.dbus.dbus-launch} --exit-with-session "$0" "$@"
+          ${config.systemd.package}/bin/systemctl --user start dbus.socket
         fi
       ''}