nixos/getty: add missing --keep-baud

systemd ships `units/serial-getty@.service.m4` with the `--keep-baud`
option.

We override that unit, and didn't add the `--keep-baud` option. (We have
it in our other getty options there).

Having `--keep-baud` in `serial-getty@` makes a lot of sense - the
console keeps working if it's initialized with a less standard baud
rate, such as the [Helios64](https://wiki.kobol.io/helios64/intro/).

nixos/modules/services/ttys/getty.nix

@@ -118,7 +118,7 @@ in
       let speeds = concatStringsSep "," (map toString config.services.getty.serialSpeed); in
       { serviceConfig.ExecStart = [
           "" # override upstream default with an empty ExecStart
-          (gettyCmd "%I ${speeds} $TERM")
+          (gettyCmd "%I --keep-baud ${speeds} $TERM")
         ];
         restartIfChanged = false;
       };