`remove_artifact_from_cache` used a hard-coded path to remove
artifacts, which wasn't sufficient for integration tests.

Tristan Maat authored Jan 02, 2019 and Tristan Maat committed Jan 18, 2019

This happened when bst is invoked with --message-lines 0 or
--error-lines 0, and was arguably a little too verbose (the user
explicitly asked us not to show them any lines, after all).

Fixes #779