Windows Path fixes

This is an alternate approach for the issues uncovered in #353, #354. It only honors the `Path` key on Windows systems when fetching the path to be prepended. We'll update `Omnibus::Software#with_embedded_path` to always return `Path` and `PATH` on Windows systems.

I believe this approach will be easier to reason about when debugging build issues.

/cc @lamont-granquist @opscode/release-engineers 

Petty, but the .to_s call here is superfluous now that .upcase is removed