allow using buildback in CI tests

[jamemackson]

This PR addresses an issue we found while attempting to use this buildpack in heroku CI where the submodule code wasn't being checked out.

in investigating it turns out the SOURCE_VERSION variable is not available in CI but HEROKU_TEST_RUN_BRANCH is so this PR checks if CI is true and if so, uses HEROKU_TEST_RUN_BRANCH variable for the checkout command else uses the existing SOURCE_VERSION variable.