grc: use installShellFiles

pkgs/tools/misc/grc/default.nix

@@ -1,6 +1,7 @@
 { lib
 , fetchFromGitHub
 , buildPythonApplication
+, installShellFiles
 }:
 
 buildPythonApplication rec {
@@ -22,10 +23,14 @@ buildPythonApplication rec {
     done
   '';
 
+  nativeBuildInputs = [ installShellFiles ];
+
   installPhase = ''
     runHook preInstall
+
     ./install.sh "$out" "$out"
-    install -Dm444 -t $out/share/zsh/vendor-completions _grc
+    installShellCompletion --zsh --name _grc _grc
+
     runHook postInstall
   '';