Don't include networkd units unless enabled

Otherwise, the enabled -> disabled transition won't be handled
correctly (switch-to-configuration currently assumes that if a unit is
running and exists, it should be restarted).

nixos/modules/system/boot/resolved.nix

@@ -18,6 +18,8 @@ with lib;
 
   config = mkIf config.services.resolved.enable {
 
+    systemd.additionalUpstreamSystemUnits = [ "systemd-resolved.service" ];
+
     systemd.services.systemd-resolved = {
       wantedBy = [ "multi-user.target" ];
       restartTriggers = [ config.environment.etc."systemd/resolved.conf".source ];