Point kernel-builder to ChipFlow fork with macOS 15 toolchain fix

The upstream kernel-builder doesn't yet have multi-strategy Metal
toolchain detection for macOS 14/15. Use our ChipFlow fork's
metal-stack branch which includes the fix (PR huggingface/kernels#308).

Co-developed-by: Claude Code v2.1.50 (claude-opus-4-6)

Author: robtaylor

flake.lock

@@ -2,7 +2,7 @@
   description = "Flake for fused RMS normalization kernel";
 
   inputs = {
-    kernel-builder.url = "github:huggingface/kernel-builder";
+    kernel-builder.url = "github:ChipFlow/kernels/metal-stack";
   };
 
   outputs =