Run fork pull requests' CI on Diffinery's runners, not GitHub's - #22

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/setup-node@v4, pnpm/action-setup@b906affcce14559ad1aafd4ab0e942779e9f58b1. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/