iamb: fix darwin build (#540772)

pkgs/by-name/ia/iamb/package.nix

@@ -6,6 +6,8 @@
   writableTmpDirAsHomeHook,
   versionCheckHook,
   nix-update-script,
+  llvmPackages,
+  stdenv,
 }:
 
 rustPlatform.buildRustPackage (finalAttrs: {
@@ -27,8 +29,17 @@ rustPlatform.buildRustPackage (finalAttrs: {
 
   nativeBuildInputs = [
     installShellFiles
+  ]
+  ++ lib.optionals stdenv.hostPlatform.isDarwin [
+    # TODO: Remove once #536365 reaches this branch
+    llvmPackages.lld
   ];
 
+  env = lib.optionalAttrs stdenv.hostPlatform.isDarwin {
+    # TODO: Remove once #536365 reaches this branch
+    NIX_CFLAGS_LINK = "-fuse-ld=lld";
+  };
+
   postInstall = ''
     installManPage $src/docs/iamb.{1,5}
     install -D $src/docs/iamb.svg -t $out/share/icons/hicolor/scalable/apps