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

[Michael Buch via llvm-commits]

Michael137 wrote:

CC @vogelsgesang @walter-erquinigo since they're more familiar with the DAP infrastructure than me. Any idea why we might fail to launch the test executable in a container (inside a container)?

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