(maint) Windows failing spec fixes

 - The usage of Dir.getwd in tests has been simplified to
   File.expand_path to properly handle mixed case CWD
 - util_spec.rb ensures 'FOO' Env var cleared before test starts
 - Windows SID checks should be performed as a regex match to
    properly handle all accounts starting with S-1-5-*, which denotes
    accounts with an identifier authority of SECURITY_NT_AUTHORITY
    instead of hardcoding S-1-5-32-544, the Administrators group