Faster Scaleway Apple Silicon runner builds via host-cached Tart VM disk reads

The host caching flag is now enabled for Tart VMs running on Scaleway Apple Silicon runners (via the tart-kubelet launchd agent in internal/tart/tart.go). Tart is now launched with caching=cached for ephemeral VM disk reads, so the host page-caches the VM disk image instead of using the slower default. In on-host A/B testing on real Scaleway M2-L (M2 Pro 10C / 16 GB) hardware, warm disk reads improved from 4.2 GB/s to 7.7 GB/s (roughly 1.8× faster), which addresses a major source of per-task overhead on CAS-heavy Xcode builds. Because the VMs are ephemeral and discarded after each Pod, host read caching has no durability tradeoff.