[COOK-4725] 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-4725

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