[bazel] Update BAZEL.md to reflect the new Mac toolchain. Bug: b/307796721 Change-Id: I79dbfeab630c137b10a55087a6c476c21abd15dc Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/771516 Auto-Submit: Leandro Lovisolo <lovisolo@google.com> Commit-Queue: Leandro Lovisolo <lovisolo@google.com> Reviewed-by: John Stiles <johnstiles@google.com>