fix: use dedicated buildx builder instance in cloud build

[cuixq]

Second try for #5692

Cloud Build steps building Go images (such as build-importer) have been intermittently failing with the following error:

ERROR: failed to solve: rpc error: code = Unknown desc = failed to solve with frontend dockerfile.v0: failed to solve with frontend gateway.v0: frontend grpc server closed unexpectedly

#5874 tried to fix the issue by pinning # syntax=docker/dockerfile:1.4:

• Even when pinned to an older version (1.4) that introduced --build-context, the fundamental mechanism still relied on spawning an ephemeral helper container over gRPC (gateway.v0).
• The underlying host daemon's runtime and memory pressure during Go builds still caused the gRPC connection to the helper container to terminate unexpectedly (frontend grpc server closed unexpectedly).

---

Instead of attempting to patch the parser on top of an outdated host daemon, we spin up a dedicated, standalone modern BuildKit daemon container:

1. Added setup-buildx step in deployment/build-and-stage.yaml:
   • Executes docker buildx create --name osv-builder --use --driver-opt network=host.
   • Starts an official moby/buildkit container with modern BuildKit and native --build-context support.
2. Shared docker-config Volume:
   • Mounts /root/.docker across all buildx steps so each step container seamlessly connects to and reuses the osv-builder instance.
3. --load Flag:
   • Exports the built images from the BuildKit container back into the local Docker daemon image store so standard docker push steps work without modification.
4. Removed # syntax= headers:
   • Removed from go/Dockerfile and go/cmd/osv-linter-worker/Dockerfile since the modern BuildKit instance handles --build-context natively without needing external gateway containers.