[libcxx] [test] Fix the temp_directory_path test for windows

Check a different set of env vars, don't check the exact value
of the fallback path. (GetTempPath falls back to returning the Windows
folder if nothing better is available in env vars.)

The test still fails one check on windows (due to relying on perms::none),
which will be addressed separately.

Differential Revision: https://reviews.llvm.org/D98139