nixos/gnunet: set private TMPDIR (#34945)

nixos/modules/services/networking/gnunet.nix

···
       after = [ "network.target" ];
       wantedBy = [ "multi-user.target" ];
       path = [ pkgs.gnunet pkgs.miniupnpc ];
+      environment.TMPDIR = "/tmp";
+      serviceConfig.PrivateTemp = true;
       serviceConfig.ExecStart = "${pkgs.gnunet}/lib/gnunet/libexec/gnunet-service-arm -c ${configFile}";
       serviceConfig.User = "gnunet";
       serviceConfig.UMask = "0007";