authelia: fix cross build (#409335)

pkgs/servers/authelia/default.nix

@@ -1,5 +1,6 @@
 {
   lib,
+  stdenv,
   nodejs,
   pnpm,
   fetchFromGitHub,
@@ -59,15 +60,17 @@ buildGoModule rec {
   # several tests with networking and several that want chromium
   doCheck = false;
 
-  postInstall = ''
-    mkdir -p $out/etc/authelia
-    cp config.template.yml $out/etc/authelia
-
-    installShellCompletion --cmd authelia \
-      --bash <($out/bin/authelia completion bash) \
-      --fish <($out/bin/authelia completion fish) \
-      --zsh <($out/bin/authelia completion zsh)
-  '';
+  postInstall =
+    ''
+      mkdir -p $out/etc/authelia
+      cp config.template.yml $out/etc/authelia
+    ''
+    + lib.optionalString (stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
+      installShellCompletion --cmd authelia \
+        --bash <($out/bin/authelia completion bash) \
+        --fish <($out/bin/authelia completion fish) \
+        --zsh <($out/bin/authelia completion zsh)
+    '';
 
   doInstallCheck = true;
   installCheckPhase = ''