caffeine-ng: fix paths in desktop files

(#73123)

pkgs/tools/X11/caffeine-ng/default.nix

@@ -23,10 +23,15 @@ python3Packages.buildPythonApplication rec {
 
   doCheck = false; # There are no tests.
 
-  postBuild = ''
+  postInstall = ''
     mkdir -p $out/share
     cp -r share $out/
+    # autostart file
+    cp -r $out/lib/python*/site-packages/etc $out/etc/
     glib-compile-schemas --strict $out/share/glib-2.0/schemas
+    for i in $(find $out -name "*.desktop"); do
+      substituteInPlace $i --replace /usr $out
+    done
   '';
 
   meta = with lib; {