delta: fix cross compilation

fixes `nix-build -A pkgsCross.aarch64-multiplatform.delta`

the pattern to use an emulator when generating shell completions is
well-established, see e.g. pkgs/by-name/st/starship/package.nix.
it evaluates to a lightweight `exec "$@"` when emulation isn't required.

pkgs/by-name/de/delta/package.nix

@@ -1,6 +1,7 @@
 {
   lib,
   rustPlatform,
+  buildPackages,
   fetchFromGitHub,
   installShellFiles,
   pkg-config,
@@ -42,12 +43,17 @@ rustPlatform.buildRustPackage (finalAttrs: {
     RUSTONIG_SYSTEM_LIBONIG = true;
   };
 
-  postInstall = ''
-    installShellCompletion --cmd delta \
-      --bash <($out/bin/delta --generate-completion bash) \
-      --fish <($out/bin/delta --generate-completion fish) \
-      --zsh <($out/bin/delta --generate-completion zsh)
-  '';
+  postInstall = lib.optionalString (stdenv.hostPlatform.emulatorAvailable buildPackages) (
+    let
+      emulator = stdenv.hostPlatform.emulator buildPackages;
+    in
+    ''
+      installShellCompletion --cmd delta \
+        --bash <(${emulator} $out/bin/delta --generate-completion bash) \
+        --fish <(${emulator} $out/bin/delta --generate-completion fish) \
+        --zsh <(${emulator} $out/bin/delta --generate-completion zsh)
+    ''
+  );
 
   # test_env_parsing_with_pager_set_to_bat sets environment variables,
   # which can be flaky with multiple threads: