auth0-cli: install shell completion

pkgs/by-name/au/auth0-cli/package.nix

@@ -2,6 +2,8 @@
   lib,
   buildGoModule,
   fetchFromGitHub,
+  installShellFiles,
+  stdenv,
 }:
 
 buildGoModule rec {
@@ -34,6 +36,15 @@ buildGoModule rec {
       --replace-fail "TestFetchUniversalLoginBrandingData" "SkipFetchUniversalLoginBrandingData"
   '';
 
+  nativeBuildInputs = [ installShellFiles ];
+
+  postInstall = lib.optionalString (stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
+    installShellCompletion --cmd auth0 \
+      --bash <($out/bin/auth0 completion bash) \
+      --fish <($out/bin/auth0 completion fish) \
+      --zsh <($out/bin/auth0 completion zsh)
+  '';
+
   subPackages = [ "cmd/auth0" ];
 
   meta = with lib; {