Commit ca58bd0a50f38de43b401df716806c0f83479a8e 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.
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