alsa-scarlett-gui: fixed desktop entry

Fixed desktop entry of the app so that it shows up in the menu.
In place search and replace was implemented in the postInstall phase
using sed.

pkgs/applications/audio/alsa-scarlett-gui/default.nix

@@ -24,6 +24,9 @@ stdenv.mkDerivation rec {
   sourceRoot = "source/src";
   nativeBuildInputs = [ pkg-config wrapGAppsHook4 ];
   buildInputs = [ gtk4 alsa-lib ];
+  postInstall = ''
+      sed -Ei "s/(Exec\s?=\s?).*/\1alsa-scarlett-gui\nTryExec=alsa-scarlett-gui/" $out/share/applications/vu.b4.${pname}.desktop
+  '';
 
   # causes redefinition of _FORTIFY_SOURCE
   hardeningDisable = [ "fortify3" ];