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.