[llvm] [lit] Fix to make "RUN: env PATH=..." work as intended (PR #165308)

[Björn Pettersson via llvm-commits]

bjope wrote:

I spent some time trying to figure out how to make a nice regression test for this. But I'm not quite sure how to.
I was aiming at adding something in utils/lit/tests, that would change the PATH to point at some Input directory with a script that it would execute. But I think it gets a bit complicated as I would need to make sure that would work on every host system etc (or find ways to limit it to Linux or something).

https://github.com/llvm/llvm-project/pull/165308