github.event.pull_request.base.sha is a snapshot taken when the PR was
last pushed, not the current tip of the base branch. A reopened or rerun
PR therefore builds with whatever master looked like at its last push,
and a tooling fix that landed on master since then never reaches it:
the daily sync PR reopened after #553 merged checked out a base without
helpers/artifact-helpers.sh and failed at "Pack artifact".
Check out base.ref instead. The plan's diff and the empty-PR check still
compare base.sha to head.sha, so the list of changed packages is
unaffected; only the tooling that runs on the droplet moves to the tip.
Ryan Hughes
2c22669d65