Symlink to /usr/bin/xcrun so that xcrun binary

is usable during build (used for compiling Metal shaders)

Fixes #6117

Author: josephst

.devops/nix/package.nix

@@ -4,6 +4,7 @@
   config,
   stdenv,
   mkShell,
+  runCommand,
   cmake,
   ninja,
   pkg-config,
@@ -87,6 +88,11 @@ let
     ]
   );
 
+  darwinSymlinks = runCommand "darwin-build-symlinks" {} ''
+    mkdir -p $out/bin
+    ln -s /usr/bin/xcrun $out/bin
+  '';
+
   # apple_sdk is supposed to choose sane defaults, no need to handle isAarch64
   # separately
   darwinBuildInputs =
@@ -96,7 +102,7 @@ let
       CoreVideo
       CoreGraphics
     ]
-    ++ optionals useMetalKit [ MetalKit ];
+    ++ optionals useMetalKit [ MetalKit darwinSymlinks ];
 
   cudaBuildInputs = with cudaPackages; [
     cuda_cccl.dev # <nv/target>
@@ -157,6 +163,8 @@ effectiveStdenv.mkDerivation (
       substituteInPlace ./*.py --replace "/usr/bin/env python" "${llama-python}/bin/python"
     '';
 
+    __noChroot = useMetalKit && effectiveStdenv.isDarwin;
+
     nativeBuildInputs =
       [
         cmake