nixos/networkd: fix networking.networkd.static test

Commit ca58bd0a50 broke the test networking.networkd.static. This happened because the test sets `networking.defaultGateway`. This is implemented by adding the gateway to the list of `routes` using `mkDefault`. The `routes` are then overridden by an empty list in the newly added code. Replace `mkDefault` with `id` so the two lists are merged and everything (hopefully) works as expected.
See https://github.com/NixOS/nixpkgs/pull/144590 for a more context.

nixos/modules/tasks/network-interfaces-systemd.nix

@@ -92,7 +92,7 @@ in
             };
           };
         });
-        networks."40-${i.name}" = mkMerge [ (genericNetwork mkDefault) {
+        networks."40-${i.name}" = mkMerge [ (genericNetwork id) {
           name = mkDefault i.name;
           DHCP = mkForce (dhcpStr
             (if i.useDHCP != null then i.useDHCP else false));