This did not seem to help with building bazel-watcher on Hydra. I believe this is still a good idea as every Bazel user on Darwin must set this variable manually to get it to work. @Profpatsch what do you think?

Can you point me to documentation about why that variable is necessary and in which cases it has to be set?

I’ve seen it in rules_haskell but the linked issue is not very enlightening.

It's not documented as far as I know, but you can see what it does in the source: https://github.com/bazelbuild/bazel/blob/71bc38fe2d165fe523b19d7879096d3fdd2086fc/tools/cpp/cc_configure.bzl#L54.

It switches between macOS specific and unix generic detection of the CPP toolchain.

I'm against setting this in the Bazel wrapper though: if someone installs Bazel via nix in a profile, it should be possible to use it outside of nix, with the Xcode or CLT toolchains.

I'd prefer having this set in buildBazelPackage.

#56033

closing in favor of #56033. Thanks, @uri-canva for your effort on this!