wmfocus: use buildFeatures

pkgs/applications/window-managers/i3/wmfocus.nix

···
   # For now, this is the only available featureset. This is also why the file is
   # in the i3 folder, even though it might be useful for more than just i3
   # users.
-  cargoBuildFlags = [ "--features i3" ];
+  buildFeatures = [ "i3" ];
 
   meta = with lib; {
     description = "Visually focus windows by label";