[libcxx] [llvm] [libc++] Make CC and CXX environment variables mandatory in run-buildbot (PR #166875)

[Louis Dionne via llvm-commits]

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ldionne wrote:

I'd like to investigate this in a follow-up, since this might require touching a bunch of lines in `run-buildbot`.

https://github.com/llvm/llvm-project/pull/166875