Match Lake's trace githash to the toolchain that built the Mathlib cache - #1

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

Failed to restore: "/usr/bin/tar" failed with error: The process '/usr/bin/tar' failed with exit code 2