zig_0_13: fix darwin SDK lookup

pkgs/development/compilers/zig/0.13/default.nix

@@ -70,12 +70,20 @@ stdenv.mkDerivation (finalAttrs: {
   # https://github.com/NixOS/nixpkgs/issues/317055#issuecomment-2148438395
   strictDeps = !stdenv.cc.isClang;
 
-  # Zig's build looks at /usr/bin/env to find dynamic linking info. This doesn't
-  # work in Nix's sandbox. Use env from our coreutils instead.
-  postPatch = ''
-    substituteInPlace lib/std/zig/system.zig \
-      --replace "/usr/bin/env" "${lib.getExe' coreutils "env"}"
-  '';
+  postPatch =
+    # Zig's build looks at /usr/bin/env to find dynamic linking info. This doesn't
+    # work in Nix's sandbox. Use env from our coreutils instead.
+    ''
+      substituteInPlace lib/std/zig/system.zig \
+        --replace "/usr/bin/env" "${lib.getExe' coreutils "env"}"
+    ''
+    # Zig tries to access xcrun at the absolute system path to query the macOS SDK
+    # location, which does not work in the darwin sandbox.
+    # We can just provide "xcrun" instead as it is part of the darwin stdenv.
+    + lib.optionalString stdenv.hostPlatform.isDarwin ''
+      substituteInPlace lib/std/zig/system/darwin.zig \
+        --replace-fail /usr/bin/xcrun xcrun
+    '';
 
   postBuild = ''
     stage3/bin/zig build langref