build_env.py chooses llvm-ar on Darwin, which isn't there.

[dr2chase]

I tried to run xed/mfile.py, but it failed with an error when it tried (unsuccessfully) to run llvm-ar. There's a perfectly good ar command in /usr/bin on macOS/Darwin, and it worked fine when I used that.

I used shutil.which(cmd) is not None (that is verbatim from an LLM, by-the-way, with no additional human input) to check for existence of cmd on $PATH, and that was helpful in avoiding commands that weren't there.