fix: move default user creation to first-setup - #422

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/upload-artifact@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Volume mount /:/run/host is going to mount '/' into the container which may cause file ownership change in the entire file system and cause Actions Runner to lose permission to access the disk.