Dart: build flute directly from dart-lang/flute with Dart 3.14.0-315.3.beta - #333

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

git diff --name-only HEAD^...HEAD