The pinned nixpkgs provides apple-sdk 26.0, but building MLX requires
SDK 26.2. The upstream package reads versions.json via a relative path
at eval time, so it can't be overridden through callPackage args.
Added a thin overlay that copies the upstream apple-sdk source and
patches only metadata/versions.json to point at SDK 26.2. Also enabled
MLX_BUILD_CPU in the MLX nix build.
This avoids vendoring the entire apple-sdk package (~2200 lines) while
still getting the SDK version we need.
Test plan:
- CI
- Built and ran on two machines connected with Thunderbolt 5 - Kimi K2.5
starts in Tensor+RDMA and seems sensible.
