improve observability and readiness timeout for buildkitd startup

[ajwerner]

Summary

Follow-up to the supabase/supabox variance investigation: a job that hit the buildkitd worker-readiness timeout fell back to a cold local builder (13m29s build) even though the daemon was still alive and serving 20 minutes later in the post step — the readiness window was simply too short, and the failure left no evidence and no backend report.

Changes (all zero-customer-change; picked up via the moving v1 tag):

• buildkitdTimeoutMs: 30s → 120s. Slow worker init (loading a large sticky-disk cache) previously triggered fallback; a cold local build + the skipped sticky-disk commit that poisons downstream runs costs far more than waiting longer.
• On worker-readiness timeout in startAndConfigureBuildkitd:
  • dump the tail of /tmp/buildkitd.log into the job log (previously this only happened in the post step when the process was gone, so slow-but-alive daemons left no evidence)
  • call reportBuildPushActionFailure("BUILDER_STARTUP", ...) so these fallbacks are visible to the backend (previously only fork failures were reported)
• BPA_BUILDKITD_READY_DURATION_MS is now reported in a finally, so timeout cases show up in the metric instead of only successes.
• Extracted the log-tail helper as logBuildkitdLogTail in setup_builder.ts (replaces the post-step-only logBuildkitdCrashLogs in main.ts).

Link to Devin session: https://app.devin.ai/sessions/f9e26ad52787476784fe0848cd40551d
Requested by: @ajwerner

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