[llvm] Allow specifying libcxx builder image. (PR #110303)

[Michael Buch via llvm-commits]

Michael137 wrote:

Interestingly, looks like @mordante has run into this before with the data-formatter CI:
* https://discourse.llvm.org/t/running-lldb-in-a-container/76801
* https://github.com/llvm/llvm-project/pull/88312

Though the fix for it doesn't seem to work for this case?

https://github.com/llvm/llvm-project/pull/110303