WIP: Make archive and announce jobs resilient to loss of CI cache

• If the cache is absent, we use the xeplist diff between the first parent of the current commit and the current commit.
• If a pipeline has been triggered, we do not send emails automatically. This is a safeguard against intentional cache erasure and re-running of a pipeline against the same commit; with the above change, that would otherwise potentially cause re-sending of emails.

Fixes lab-infrastructure#1.