[libcxx] [test] Don't add dirs from the LIB env var to PATH

The directories in LIB normally only contain import libraries or
static libraries, no runtime DLLs that would need to be found
while running tests.

This code stems from 1cd196e7,
which (among other things) tried to do this:

> * [Test] Fix handling of library runtime search paths by correctly adding them
>   to the PATH variable when running the tests.

It's unclear to me exactly what this fixed (or tried to) at the time,
as the LIB var doesn't normally point to runtime libs.

Differential Revision: https://reviews.llvm.org/D99241