nixos-module/container/anon: make core flags optional

nix/nixos-module/container/anon.nix

@@ -64,7 +64,7 @@ in
     } ];
   }) tunnels;
 
-  systemd.network.networks = {
+  systemd.network.networks = lib.mkIf enabled {
     # Wireguard transported through another routing table
     # (containing upstream by bird ospf)
     core.routingPolicyRules = [ {