Disable TestEnvironment on Windows

The input source file seems to be triggering an error in the Visual
Studio headers.

> xstddef:338:2: error: ''auto' return without trailing return type;
> deduced return types are a C++14 extension

I tried converting the test to use the %build stuff Zachary added, but
that seems to be missing some Darwin support. Disabling the test on
Windows in the meantime.

llvm-svn: 360624