[llvm] [lit] Fix missing command for external Windows shell (PR #116603)

[Sven van Haastregt via llvm-commits]

svenvh wrote:

> Why are you using the external shell on Windows? The internal is the default and we are aiming to get rid of external shell support entirely, as I understand it.

We are indeed migrating our (downstream) tests away from the external shell but we're not ready yet to drop it entirely.  I wasn't aware of any removal plans of external shell support, but that seems like the right longer term direction.

https://github.com/llvm/llvm-project/pull/116603