Every push to a PR rebuilt every package the PR touches, on every
architecture, even when only one of them changed. The daily sync PR
carries around thirty package/arch pairs; fixing one package meant
rebuilding all of them, and an aarch64 build under QEMU takes up to an
hour. Nine runs of that PR cost about 36 droplet-hours in two days.
The planner now asks the artifact store for <pkg>-<arch>-<tree hash>
before adding an entry to the matrix and drops entries that already
have one. That is the same lookup publish.yml makes on merge, so a
reused entry publishes exactly the file it would have anyway. Dry run
against the current sync PR: 27 of 31 entries reused, 4 built.
Pack and Upload no longer run with always(): only a successful build
uploads, so an artifact's existence means that tree built.
workflow_dispatch always builds; it is an explicit request.
Ryan Hughes
c74c708f35