(CFACT-121) Fix check for executable file

Corrected a typo: the environment variable is PATHEXT (not EXTPATH).
Digging through the Ruby source code shows that File.executable? eventually
just checks that the file is a directory, or a file with the extension
bat, .cmd, .com, .exe. So we re-use extpath_helper to implement the original
intent of the Facter execution::which method.