ci/build-nightly-ffmpeg-windows

[2391384896]

Summary

• Build the fixed Windows x64 FFmpeg bundle in the Nightly job with the pinned MSYS2 action before packaging.
• Pass FFMPEG_BUNDLE_SOURCE_DIR into forge:make, matching the Release workflow contract merged in ci/packaged-agent-toolchain-assets #32.
• Keep the change scoped to the Windows app job; test launcher jobs remain unchanged.

Validation

• Ruby YAML parse
• actionlint v1.7.12
• git diff --check
• Release workflow equivalent validated by https://github.com/floatboatai/floatboat-release/actions/runs/29491544866