nixos-module.nix: add timer

nixos-module.nix

@@ -99,6 +99,13 @@ in
         Group = cfg.group;
       };
     };
+    systemd.timers.ticker-update = {
+      wantedBy = [ "multi-user.target" ];
+      partOf = [ "ticker-update.service" ];
+      timerConfig = {
+        OnCalendar = cfg.updateInterval;
+      };
+    };
 
     systemd.services.ticker-serve = {
       wantedBy = [ "multi-user.target" ];