fix: missing guard for NO_XIPH_LIBS in mixer_output and in some tests - #528

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, invertase/github-action-dart-analyzer@v3. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/