[libc++][test] Propagate host environment to libc++ test suite

Propagate the complete host environment to the tests run via the new
testconfig. This ensures that all envvars needed e.g. for the compiler
to work correctly are present. This mimics the behavior explicitly
implemented in the legacy config.

https://github.com/llvm/llvm-project/issues/56816

Differential Revision: https://reviews.llvm.org/D130843