flake.lock: Update (26.05 -> unstable) - #46

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: DeterminateSystems/magic-nix-cache-action@565684385bcd71bad329742eefe8d12f2e765b39, DeterminateSystems/nix-installer-action@c5a866b6ab867e88becbed4467b93592bce69f8a. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/