nixos/nixos-container: Always apply extraVeth ip configuration

Fixes that `containers.<name>.extraVeths.<name>` configuration was not
always applied.

When configuring `containers.<name>.extraVeths.<name>` and not
configuring one of `containers.<name>.localAddress`, `.localAddress6`,
`.hostAddress`, `.hostAddress6` or `.hostBridge` the veth was created,
but otherwise no configuration (i.e. no ip) was applied.

nixos-container always configures the primary veth (when `.localAddress`
or `.hostAddress` is set) to be the containers default gateway, so
this fix is required to create a veth in containers that use a different
default gateway.

To test this patch configure the following container and check if the
addresses are applied:
```
  containers.testveth = {
    extraVeths.testveth = {
      hostAddress = "192.168.13.2";
      localAddress = "192.168.13.1";
    };
    config = {...}:{};
  };
```

nixos/modules/virtualisation/nixos-containers.nix

@@ -56,9 +56,9 @@ let
             ip -6 route add $HOST_ADDRESS6 dev eth0
             ip -6 route add default via $HOST_ADDRESS6
           fi
+        fi
 
         ${concatStringsSep "\n" (mapAttrsToList renderExtraVeth cfg.extraVeths)}
-        fi
 
         # Start the regular stage 1 script.
         exec "$1"
@@ -223,8 +223,8 @@ let
             ${ipcall cfg "ip route" "$LOCAL_ADDRESS" "localAddress"}
             ${ipcall cfg "ip -6 route" "$LOCAL_ADDRESS6" "localAddress6"}
           fi
-          ${concatStringsSep "\n" (mapAttrsToList renderExtraVeth cfg.extraVeths)}
         fi
+        ${concatStringsSep "\n" (mapAttrsToList renderExtraVeth cfg.extraVeths)}
       ''
   );