nixos: Add nm-openvpn to the networkmanager group

This is to satisfy the polkit restriction limiting
org.freedesktop.NetworkManager.* dbus messages to members of that
group.

Should help with #24806

nixos/modules/services/networking/networkmanager.nix

@@ -241,6 +241,7 @@ in {
     users.extraUsers = [{
       name = "nm-openvpn";
       uid = config.ids.uids.nm-openvpn;
+      extraGroups = [ "networkmanager" ];
     }];
 
     systemd.packages = cfg.packages;