Authenticate FFmpeg metadata requests

[FaulMit]

Summary

• authenticate FFmpeg release metadata requests when GITHUB_TOKEN is available
• pass the scoped Actions token from CI, GitHub Release, and Microsoft Store build workflows
• retain unauthenticated fallback for local builds without GitHub credentials

Cause

The latest main build exhausted the shared unauthenticated GitHub API rate limit while resolving the current FFmpeg 8.1 asset.

Verification

• authenticated FFmpeg runtime acquisition with SHA-256 verification
• Release build: 0 warnings, 0 errors
• git diff --check