log buildkitd worker readiness wait duration in setup step

[ajwerner]

Summary

The time spent waiting for buildkitd workers to become ready was only reported as a backend metric (BPA_BUILDKITD_READY_DURATION_MS), invisible in the job log. This adds the elapsed wait to the existing "Found N workers" log line in startAndConfigureBuildkitd, so users can see how long the daemon took to come up directly in the setup step output:

Found 1 workers, required 1 (buildkitd workers ready after 4321ms)

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