fix escaping of powershell variable

As this is nested in a powershell variable $command, it must be escaped
otherwise it is evaluated when the variable is created, giving an error that
"The term 'True' is not recognized as the name of a cmdlet, function,
script". This prevented using a puppet.working_directory on Windows.
Merge pull request #4059 from brettporter/windows-puppet-wd

Unable to use puppet.working_directory on Windows