hyprshade: Fix unavailable default shaders

The orignal code looks for the shaders in either the Python data
directory, which seems to not be working well under the way nixpkgs
installs Python builds.
There is the environment variable HYPRSHADE_SHADERS_DIR in
https://github.com/loqusion/hyprshade/blob/main/src/hyprshade/shader/dirs.py#L12
that lets us configure it to "$out/share/hyprshade/shaders" instead to
give full functionality.

pkgs/applications/window-managers/hyprwm/hyprshade/default.nix

@@ -24,6 +24,10 @@ buildPythonPackage rec {
 
   propagatedBuildInputs = [ more-itertools click ];
 
+  postFixup = ''
+    wrapProgram $out/bin/hyprshade --set HYPRSHADE_SHADERS_DIR $out/share/hyprshade/shaders
+  '';
+
   meta = with lib; {
     homepage = "https://github.com/loqusion/hyprshade";
     description = "Hyprland shade configuration tool";