Handle self.outPath when it is available + improve real flake use - #18

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@v13. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/