git-town: fix cross build (#406782)

pkgs/by-name/gi/git-town/package.nix

@@ -1,5 +1,6 @@
 {
   lib,
+  stdenv,
   buildGoModule,
   fetchFromGitHub,
   installShellFiles,
@@ -64,12 +65,14 @@ buildGoModule rec {
     in
     [ "-skip=^${builtins.concatStringsSep "$|^" skippedTests}$" ];
 
-  postInstall = ''
+  postInstall =
+    lib.optionalString (stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
       installShellCompletion --cmd git-town \
         --bash <($out/bin/git-town completions bash) \
         --fish <($out/bin/git-town completions fish) \
         --zsh <($out/bin/git-town completions zsh)
-
+    ''
+    + ''
       wrapProgram $out/bin/git-town --prefix PATH : ${lib.makeBinPath [ git ]}
     '';