Fix t/op/taint.t on Windows

$ENV{PATH} seems to be the problem.  If we clear it, then we can’t
spawn another process.  I am basing this solely on this comment ear-
lier in the file:

    # On Windows we can't spawn a fresh Perl interpreter unless at
    # least the Windows system directory (usually C:\Windows\System32)
    # is still on the PATH.  There is however no way to determine the
    # actual path on the current system without loading the Win32
    # module, so we just restore the original $ENV{PATH} here.