test: assert the nix runner backend in CI (#49) - #55

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@11d5960a326750d5838078e36cf38b85af677262, actions/setup-go@40f1582b2485089dde7abd97c1529aa768e1baff. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/