qt5.15: restore the version override

I am bad at this.

pkgs/development/libraries/qt-5/5.15/srcs.nix

···
 
 let
   version = "5.15.5";
-  overrides = {};
+  overrides = {
+    qtscript.version = "5.15.4";
+  };
 
   mk = name: args:
     let