This allows executables to be found by :!, system(), and executable() if
they live next to ("sibling" to) nvim.exe. This is what gvim on Windows
does, and also matches the behavior of Win32 SearchPath().
c4a249a736/src/os_win32.c (L354-L370)
- In UNIX systems where unsetenv() is available, it is used. Otherwise
the variables are set with the empty string.
- New check HAVE_UNSETENV for unsetenv()
- Added unit test to env_spec.lua