[llvm] Fix ExecuteAndWait with empty environment on Windows (PR #158719)

[Paul Osmialowski via llvm-commits]

pawosm-arm wrote:

> Maybe disable the test like #160277?

Locally, we do this: `sed -i "s/ifndef __MINGW32__/ifndef LLVM_ON_UNIX/" llvm/unittests/Support/ProgramTest.cpp`


https://github.com/llvm/llvm-project/pull/158719