ktlint: add gnused to the script's path

As of ktlint 0.44.0, the startup script makes use of `sed`, if the latter is not available in the path, the program fails silently.

https://github.com/pinterest/ktlint/commit/f72be11d214e6b809f9977ef175e4162e7197541

pkgs/development/tools/ktlint/default.nix

···
-{ lib, stdenv, fetchurl, makeWrapper, jre_headless }:
+{ lib, stdenv, fetchurl, makeWrapper, jre_headless, gnused }:
 
 stdenv.mkDerivation rec {
   pname = "ktlint";
···
   '';
 
   postFixup = ''
-    wrapProgram $out/bin/ktlint --prefix PATH : "${jre_headless}/bin"
+    wrapProgram $out/bin/ktlint --prefix PATH : "${lib.makeBinPath [ jre_headless gnused ]}"
   '';
 
   meta = with lib; {