Make routes config explicit.

This helps Hetzner Cloud, which doesn't populate routes by
default and thus network doesn't work.

Fixes https://github.com/elitak/nixos-infect/issues/25

nixos-infect

@@ -105,6 +105,8 @@ EOF
         ipv6.addresses = [$(for a in "${eth0_ip6s[@]}"; do echo -n "
           $a"; done)
         ];
+        ipv4.routes = [ { address = "${gateway}"; prefixLength = 32; } ];
+        ipv6.routes = [ { address = "${gateway6}"; prefixLength = 32; } ];
       };
       $interfaces1
     };