nixos/mosquitto: use networkd for tests

the non-networkd backend does not wait for slaac to finish (ie, ipv6
addresses coming out of tentative state), and that breaks the mosquitto
bind_interface test slightly. if slaac takes too long the test will run
into mosquitto restart limits and fail.

nixos/tests/mosquitto.nix

@@ -66,6 +66,7 @@ in {
   in {
     server = { pkgs, ... }: {
       networking.firewall.allowedTCPPorts = [ port tlsPort anonPort ];
+      networking.useNetworkd = true;
       services.mosquitto = {
         enable = true;
         settings = {