Expand environment variables in path

This should help ease the pain from
https://github.com/opscode/chef/pull/1876
until we do another chef client release
It's not 100% clear this will work. The "official" way to expand environment variables in Windows is to call [ExpandEnvironmentStrings](http://msdn.microsoft.com/en-us/library/windows/desktop/ms724265(v=vs.85).aspx) -- probably have to choose between calling the W or A version, and see if we are using win32-api or ffi to import system calls.