signal-desktop: remove absolute path in desktop entry

Signed-off-by: Sefa Eyeoglu <contact@scrumplex.net>

pkgs/applications/networking/instant-messengers/signal-desktop/generic.nix

@@ -237,7 +237,7 @@ stdenv.mkDerivation rec {
 
     # Fix the desktop link
     substituteInPlace $out/share/applications/${pname}.desktop \
-      --replace-fail "/opt/${dir}/${pname}" $out/bin/${pname} \
+      --replace-fail "/opt/${dir}/${pname}" ${meta.mainProgram} \
       --replace-fail "StartupWMClass=Signal" "StartupWMClass=signal"
 
     # Note: The following path contains bundled libraries: