macosx nighly builds are failing

[proppy]

See https://github.com/google/xls/actions/workflows/nightly-macos.yml Possibly related to: https://github.com/bazelbuild/bazel/issues/18762 Tentative fix: bump https://github.com/bazel-contrib/toolchains_llvm ?

[cdleary]

Was playing on my home laptop and bumping it up didn't work, I tried to make the LLVM toolchains optionally register because it seems like things work "just fine" when you don't use the hermetic toolchain. Made #1378 but I'm not sure I got the rule right yet to do the conditional registration.