[libcxx] [llvm] [libc++][CI] Add hermetic Linux premerge builder (PR #226358)

[via llvm-commits]

zeroomega wrote:

> This setup with an additional container image seems a bit fragile. If someone accidentally adds an additional dependency or something there, you'll never catch it. Needing an additional container image for this is also a bit annoying.
> 
> Also, do you have examples of previous patches that have caused issues with such configurations?

This is added to catch cases like https://github.com/llvm/llvm-project/pull/224192 . That patch introduce implicit dependency on cmake on PATH instead of inherit build config from LLVM itself. 

I agree on the fragile part, but not from container, but from the buildbot side, if someone add a new implicit dependency on something there, we won't catch it. Alternatively, we can change the original linux container so all libcxx tests will be hermetic, which should be like that from the beginning. But doing that could probably break some existing premerge builders. 

https://github.com/llvm/llvm-project/pull/226358