no-x-libs.nix: Ensure that dbus doesn't use X11

It appears that packageOverrides no longer overrides aliases, so
aliases like

  dbus_tools = self.dbus.out;
  dbus_daemon = self.dbus.daemon;

now use the old, non-overriden version of dbus. That seems like a
pretty serious regression in general, but for this particular problem,
I've fixed it by replacing dbus_daemon by dbus.daemon and dbus_tools
by dbus.