Use windows_path to set the PATH

Using `execute` to alter the SYSTEM PATH has lots of negative side
effects:

1. It doesn't have a guard, so it adds the PATH every time the cookbook
is run.
2. It clobbers any previously run `windows_path` because the environment
hasn't been reloaded.

This closes https://tickets.opscode.com/browse/COOK-1765

Ticket: COOK-1765