[llvm] [flang-rt] Enable lit internal shell by default (PR #156095)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> Does it work also on windows?

The internal shell is the default on Windows, so this change should not have any impact on the Windows builds (theoretically).

https://github.com/llvm/llvm-project/pull/156095