add /usr/bin to path for building MetalKit with xcrun

Fixes #6117

Author: josephst

.devops/nix/package.nix

@@ -157,6 +157,11 @@ effectiveStdenv.mkDerivation (
       substituteInPlace ./*.py --replace "/usr/bin/env python" "${llama-python}/bin/python"
     '';
 
+    # requires sandboxing to be off or relaxed on MacOS (off by default)
+    preConfigure = lib.optionals useMetalKit ''
+      export PATH=/usr/bin:$PATH # make sure /usr/bin/xcrun is on PATH
+    '';
+
     nativeBuildInputs =
       [
         cmake