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

[Aiden Grossman via cfe-commits]

boomanaiden154 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.

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