notation: provide completions when cross-compiling

pkgs/by-name/no/notation/package.nix

@@ -1,10 +1,12 @@
 {
   lib,
-  stdenv,
   buildGoModule,
   fetchFromGitHub,
   installShellFiles,
   testers,
+
+  stdenv,
+  buildPackages,
 }:
 
 buildGoModule (finalAttrs: {
@@ -36,12 +38,20 @@ buildGoModule (finalAttrs: {
     "-X github.com/notaryproject/notation/internal/version.BuildMetadata="
   ];
 
-  postInstall = lib.optionalString (stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
-    installShellCompletion --cmd notation \
-      --bash <($out/bin/notation completion bash) \
-      --fish <($out/bin/notation completion fish) \
-      --zsh <($out/bin/notation completion zsh)
-  '';
+  postInstall =
+    let
+      exe =
+        if stdenv.buildPlatform.canExecute stdenv.hostPlatform then
+          "${placeholder "out"}/bin/${finalAttrs.meta.mainProgram}"
+        else
+          lib.getExe buildPackages.notation;
+    in
+    ''
+      installShellCompletion --cmd ${finalAttrs.meta.mainProgram} \
+        --bash <(${exe} completion bash) \
+        --fish <(${exe} completion fish) \
+        --zsh <(${exe} completion zsh)
+    '';
 
   passthru.tests.version = testers.testVersion {
     package = finalAttrs.finalPackage;