Sources advertising no prestage key and not needing previous sources should skip tracking

When adding 33d62860 it seemed that sources with BST_NO_PRESTAGE_KEY = True and BST_REQUIRES_PREVIOUS_SOURCES = False can skip tracking jobs to avoid re-calculating the source key. For sources without a pre-stage key and requiring previous sources it might be possible to cache the calculated source key in some way to communicate it to the build job.