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

[via llvm-commits]

zeroomega wrote:

The current implementation is less hermetic than our downstream builder even for linux-x64 runtimes. However, due to https://github.com/llvm/llvm-project/issues/226351 , we cannot currently building the exact same config in the libcxx ci yet.
We have tested the setup in this PR end to end and confirm it catches the catches cases like https://github.com/llvm/llvm-project/pull/224192  . This new build config should still provide additional coverage on changes that introduce implicit dependencies on host tools.

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