Use the internal lit shell

[bogner]

The external lit shell is deprecated as of llvm/llvm-project#203689

[bob80905]

I suppose that variable isn't set in the macos env, so not False = true, and that triggers the error 🤔

[bogner]

I suppose that variable isn't set in the macos env, so not False = true, and that triggers the error 🤔

It defaults to true on windows, which is why most of our CI is happy. I guess the lit config variable will go away at some point.