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

[via llvm-commits]

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zeroomega wrote:

Yes. The new dockerfile is just for creating a clean image that do not have llvm dependencies in the PATH. If you are OK to change existing premerge task to be hermetic. We can do that.

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