Build bundles snapshot machine metrics under the sampler lock

Build-processing runs no longer crash with bad-CRC machine_metrics.jsonl errors: the CLI now snapshots the metrics file under the sampler daemon’s shared lock (the same lock convention the metrics reader already used) before copying it into the upload archive, so the snapshot is always consistent with respect to the launchd writer. Bad-CRC failures from TUIST-10W are no longer produced.