get the correct path_key from the ENV on win

closes #353
This is what we used to do, e.g.:

https://github.com/opscode/omnibus-chef/blob/c73ae02c28270536acad67fb604285155bbdd64a/config/software/chef-windows.rb#L69

I took it out because it seemed crazy, but it solves this problem.  And since there shouldn't ever be two keys that match (it should be either 'PATH' or 'Path', not both), the hash key ordering issue doesn't matter as an edge condition.

So, I totally agree'd with you, but then I tried something else and it broke...