fix: wait for GPU latest tag convergence

[bradhilton]

Summary

• wait for Kubernetes pulls of the mutable GPU tag to resolve to the just-pushed digest before node prewarm
• verify each one-shot and steady prewarm pod refreshed latest to the pushed digest via initContainer imageID
• fail before SkyPilot smoke if latest is still resolving to an older digest

Testing

• bash -n scripts/build-gpu-image.sh
• scripts/build-gpu-image.sh --help
• git diff --check