key-mon #11183: use $GDK_PIXBUF_MODULE_FILE with makeWrapperArgs

This is more robust, since it won't be affected when the path to
gdk-pixbuf loaders.cache changes. Thanks @domenkozar.

pkgs/applications/video/key-mon/default.nix

···
 
   doCheck = false;
 
-  makeWrapperArgs = ''
-    --set GDK_PIXBUF_MODULE_FILE ${librsvg}/lib/gdk-pixbuf-2.0/2.10.0/loaders.cache
+  preFixup = ''
+      export makeWrapperArgs="--set GDK_PIXBUF_MODULE_FILE $GDK_PIXBUF_MODULE_FILE"
   '';
 
   meta = with stdenv.lib; {