safechecker-plugin: adapt for a 9.2 base carrying the backported 9.3 assumptions API (rocq#22164) - #2

Action failed with error: Error: Unable to locate executable file: cachix. Please verify either the file path exists or the file can be found within a directory specified by the PATH environment variable. Also check the file mode to verify the file is executable.

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v2, cachix/cachix-action@v10, cachix/install-nix-action@v16. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/