[clang] [llvm] [Clang] Enable lit internal shell by default (PR #158465)

[Oliver Hunt via llvm-commits]

ojhunt wrote:

> You have to prefix the invocation with `env`. e.g., `env ENV_VARIABLE=some_value ./command_that_reads_env_variable`.
> 
> `export` should also work for when you need an env variable to span multiple commands.

I found it eventually :D

Is there a doc page for this? (grepping for "lit" was not effective :D)

https://github.com/llvm/llvm-project/pull/158465