Update modalkit::env::mixed to mirror vim/emacs modules - #213

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v3, actions/checkout@v3, mozilla-actions/sccache-action@v0.0.9. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/