post: skip sticky disk cleanup when this step reused an existing buildkitd (release/v2) - #137

Summary

release/v2 port of #136.

When setup-docker-builder runs more than once in the same job (e.g. a composite action that retries a build and calls setup + build-push per attempt), only the first instance starts buildkitd and mounts /var/lib/buildkit. Later instances detect the running daemon and reuse it (builderMode=existing), and their post step correctly skips shutdown via maybeShutdownBuildkitd() — but it then falls straight into the sync / umount /var/lib/buildkit block while buildkitd is still alive. That unmount fails 3× with target is busy and is reported as BUILDER_CLEANUP, feeding blacksmith_stickydisk_failures_total and the "Docker Setup High Failure Count" alert. Post steps run in reverse order, so every reuse instance emits a spurious failure before the owning instance's post finally shuts down and unmounts successfully.

Fix: in the post step, if stateHelper.getBuilderMode() === "existing", skip the whole cleanup + commit-gating block. Unlike v1, v2 already records this mode in state, so no new state key is needed. The structured teardown report is still shipped for that instance (with commitSkipReason=NO_EXPOSE, as today) so the EXISTING builder mode remains visible in telemetry.

if (stateHelper.getBuilderMode() === "existing") {
  core.info("This instance reused a builder started by an earlier setup-docker-builder step; skipping cleanup ...");
  removeTmpDir();
  setCommitSkipped(lifecycle, CommitSkipReason.NO_EXPOSE);
  await shipTeardownReport(lifecycle, exportedBuilds, exposeId);
  reporter.closeBlacksmithAgentClient();
  return;
}

removeTmpDir() and shipTeardownReport() are extracted from the inline post-step code so the early-return path can reuse them. dist/ regenerated via the pre-commit hook.

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Requested by: @bruce-y

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