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. The an all uppercase key of `PATH` is always used in the hash returned by `Omnibus::Software#with_embedded_path`. I believe this approach will be easier to reason about when debugging build issues.

/cc @lamont-granquist @opscode/release-engineers