Build: Run MixinTests in CI. - #144

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, actions/setup-java@v4, gradle/actions/setup-gradle@af1da67850ed9a4cedd57bfd976089dd991e2582. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Failed to restore gradle-home-v1|Linux-X64|test[04d8ed242d6e4f9366abd4632caf0523]-cafc494812f345fcfaa23a95acf64d5dc6f0c56f: Error: Cache service responded with 400