df: simplify fn find_mount_point - #13916

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: Vampire/setup-wsl@v6.1.0. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/