Fix: NTP-sync macOS runner clock before the job starts so step and metrics timelines align

Runner metrics charts on macOS now line up with the GitHub step timeline from the start of the job. A synchronised NTP step (sntp -sS) runs on the guest right after dispatch succeeds and before the runner and metrics sampler start, so steps and metric samples share the same corrected clock instead of drifting as timed converges mid-run. Best-effort, macOS only (Linux kata VMs already get an accurate paravirt clock). Applies to VMs booted from the next runner-image roll.