Use VeriFast 26.10 for nightly-2026-09-25 - #714

PR needs 2 approvals from committee members, but it has 1

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v2, actions/github-script@v6. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/