feat: use pre-installed buildx if available and matches specified verison

[taha-au]

Note

Skips downloading buildx if a matching version is already installed, validates the requested version, and retries GitHub release downloads with exponential backoff.

• Buildx setup:
  • Prefer pre-installed buildx when available and its version matches the requested one; otherwise download and install.
  • Validate buildx-version input (latest or v<MAJOR>.<MINOR>.<PATCH>), falling back to default when invalid.
  • Add exponential backoff retry for GitHub Releases download (handles 429).
  • Always print resolved Buildx version after setup.
• Packaging:
  • Update compiled dist/index.js and source map.

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