[llvm] [lit] Use text mode when piping output (PR #217434)

[Prasoon Kumar via llvm-commits]

https://github.com/prasoon054 commented:

I think the cleanest direction is to keep the fix confined to `as_binary_reader` in `ShellEnvironment.py` gated by a platform check the same way cat.py already gates its z/OS file-read path.

https://github.com/llvm/llvm-project/pull/217434