feat(ci): add driver input to setup-buildx action

[jtoelke2]

Summary

Add a driver input to .github/actions/setup-buildx/action.yml. Default stays remote, preserving current behavior for all 7 call sites. New local option selects the docker-container driver for callers that need buildx on nv-gha-runners (no EKS BuildKit). Pure-additive prep for the OS-127 Phase 3 shadow workflow.

Related Issue

OS-49 runner migration, Phase 3 / OS-127. This is PR 2 (prep). PR 3 (shadow workflow using driver: local + GHA cache) follows once this lands.

Changes

• .github/actions/setup-buildx/action.yml:
  • New driver input with default: remote.
  • Setup step split into two conditional steps:
    • if: inputs.driver == 'remote' — unchanged wiring to the in-cluster BuildKit endpoints (tcp://buildkit-{amd64,arm64}.buildkit:1234).
    • if: inputs.driver == 'local' — new, docker/setup-buildx-action@v3 with driver: docker-container, no endpoints.
  • Existing inputs (amd64-endpoint, arm64-endpoint, name) unchanged; description clarified as remote-only.

No caller updated. Default resolution keeps all 7 existing callers (ci-image.yml, docker-build.yml, release-tag.yml ×3, release-vm-dev.yml ×2, release-dev.yml ×2) on the remote path.

Testing

• mise run pre-commit passes
• Unit tests added/updated — N/A; composite GitHub Action
• E2E tests added/updated — N/A
• Remote-driver path exercised by every Rust/Docker CI job on this PR (7 call sites)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated — N/A; plan lives in Linear OS-127 (doc pending; MCP create_document is currently returning Tool not found — will attach the plan doc once it recovers or fall back to a Linear comment)