xfce4-whiskermenu-plugin: the xfce4-panel binary is in ${xfce4panel.out}

pkgs/desktops/xfce/panel-plugins/xfce4-whiskermenu-plugin.nix

···
 
   preFixup = ''
     substituteInPlace $out/bin/xfce4-popup-whiskermenu \
-      --replace $out/bin/xfce4-panel ${xfce4panel}/bin/xfce4-panel
+      --replace $out/bin/xfce4-panel ${xfce4panel.out}/bin/xfce4-panel
   '';
 
   meta = {