Don't ask for a builder to rebuild what is already published (#870)

The split in #869 sent every tree without an artifact to the rebuild job,
so a dispatch whose x86 halves were already published waited eight minutes
for eleven droplets that each found nothing to build in five seconds. Make
the publish job's plan check in the changes job, on its hosted runner,
before a builder is requested.

.github/workflows/publish.yml

@@ -71,8 +71,17 @@ jobs:
             found=$(curl -fsS -H "Authorization: Bearer $GH_TOKEN" -H "Accept: application/vnd.github+json" \
               "https://api.github.com/repos/${{ github.repository }}/actions/artifacts?name=$label&per_page=5" \
               | jq -r '[.artifacts[] | select(.expired|not)] | sort_by(.created_at) | last | select(. != null) | "run \(.workflow_run.id), expires \(.expires_at)"')
+            # The plan the publish job makes, made here before a runner is
+            # asked for: a tree the channel already holds at master's version
+            # (a re-run, or a dispatch naming a package that is fine) needs
+            # no build, and an x86 rebuild would wait minutes for a droplet
+            # to find that out in seconds.
+            plan=""
+            [[ -n "$found" ]] || plan=$(bin/build --dry-run --mirror edge --arch "$arch" --package "$package" 2>&1 | grep -E '^==> Plan complete' || true)
             if [[ -n "$found" ]]; then
               decision="reuse the build artifact ($found)"
+            elif [[ -n "$plan" && -z "$(sed -E 's/.*would build: *//' <<<"$plan" | tr -d '[:space:]')" ]]; then
+              decision="already published at master's version, nothing to build"
             elif [[ $arch == aarch64 ]]; then
               decision="no build artifact: rebuild natively on ubuntu-24.04-arm"
               rebuild+=("$(jq -c '. + {runner: "[\"ubuntu-24.04-arm\"]"}' <<<"$entry")")