Allow integration test scripts to pass in buildpack zips
Allow integration test scripts to pass in buildpack zips

- Pass in file paths as env vars
- paths default to  for typical behavior

Co-authored-by: Zander Mackie <320db42c0fea4ffbf0ae28d5036748df300aa745@pivotal.io>

cloudfoundry/python-buildpack,cloudfoundry/python-buildpack,cloudfoundry/python-buildpack