pet: install zsh completion file

pkgs/development/tools/pet/default.nix

@@ -1,4 +1,4 @@
-{ buildGoModule, fetchFromGitHub, lib }:
+{ buildGoModule, fetchFromGitHub, installShellFiles, lib }:
 
 buildGoModule rec {
   pname = "pet";
@@ -17,6 +17,15 @@ buildGoModule rec {
 
   subPackages = [ "." ];
 
+  nativeBuildInputs = [
+    installShellFiles
+  ];
+
+  postInstall = ''
+    installShellCompletion --cmd pet \
+      --zsh ./misc/completions/zsh/_pet
+  '';
+
   meta = with lib; {
     description = "Simple command-line snippet manager, written in Go";
     homepage = "https://github.com/knqyf263/pet";