nixos/autorandr: added new KillMode

Prevents Udev Rule from killing processes started by autorandr

nixos/modules/services/misc/autorandr.nix

···
         ExecStart = "${pkgs.autorandr}/bin/autorandr --batch --change --default ${cfg.defaultTarget}";
         Type = "oneshot";
         RemainAfterExit = false;
+        KillMode = "process";
       };
     };