cert-viewer: fix cross-compilation

Tested with `nix-build -A pkgsCross.armv7l-hf-multiplatform.cert-viewer`.

Fixes: 97faa99d9d ("cert-viewer: install manual page")

pkgs/by-name/ce/cert-viewer/package.nix

@@ -1,7 +1,9 @@
 { buildGoModule
+, buildPackages
 , fetchFromGitHub
 , lib
 , installShellFiles
+, stdenv
 }:
 
 buildGoModule rec {
@@ -21,10 +23,17 @@ buildGoModule rec {
     installShellFiles
   ];
 
-  postInstall = ''
-    $out/bin/cert-viewer --help-man > cert-viewer.1
-    installManPage cert-viewer.1
-  '';
+  postInstall =
+    let
+      prog =
+        if stdenv.buildPlatform.canExecute stdenv.hostPlatform
+        then "$out/bin/cert-viewer"
+        else lib.getExe buildPackages.cert-viewer;
+    in
+      ''
+        ${prog} --help-man > cert-viewer.1
+        installManPage cert-viewer.1
+      '';
 
   meta = {
     description = "Admin tool to view and inspect multiple x509 Certificates";