Faster macOS Apple Silicon runner provisioning

macOS Apple Silicon runner provisioning now reuses a per-host “golden” base VM instead of re-downloading the Xcode image on every job recycle. Runner VMs are cloned locally via APFS clonefile (no network), so provisioning drops from roughly 7 minutes per job to seconds, and redundant image pulls (~14 TB/day across the fleet) are eliminated. In practice the Apple Silicon pool can run more concurrent builds at steady state, so queued macOS jobs get picked up sooner.