nixos/toxvpn: replace systemd script with ExecStart

Also replace `preStart` with `RuntimeDirectory`, which does the same
thing in this case.

nixos/modules/services/networking/toxvpn.nix

@@ -43,20 +43,19 @@ with lib;
       wantedBy = [ "multi-user.target" ];
       after = [ "network.target" ];
 
-      preStart = ''
-        mkdir -p /run/toxvpn || true
-        chown toxvpn /run/toxvpn
-      '';
-
-      path = [ pkgs.toxvpn ];
-
-      script = ''
-        exec toxvpn -i ${config.services.toxvpn.localip} -l /run/toxvpn/control -u toxvpn -p ${toString config.services.toxvpn.port} ${
-          lib.concatMapStringsSep " " (x: "-a ${x}") config.services.toxvpn.auto_add_peers
-        }
-      '';
-
       serviceConfig = {
+        ExecStart =
+          let
+            args = lib.cli.toCommandLineShellGNU { } {
+              i = config.services.toxvpn.localip;
+              l = "/run/toxvpn/control";
+              u = "toxvpn";
+              p = config.services.toxvpn.port;
+              a = config.services.toxvpn.auto_add_peers;
+            };
+          in
+          "${lib.getExe pkgs.toxvpn} ${args}";
+        RuntimeDirectory = "toxvpn";
         KillMode = "process";
         Restart = "on-success";
         Type = "notify";