Update Nix flake to pin Node and Rust versions

[muni-corn]

Pins nixpkgs, Node (18.16.1), and Rust (1.74.1) to versions compatible with this repository.

nixpkgs is pinned to an older version for Node v18 to build without failure.

[github-actions]

PR Reviewer Guide 🔍

⏱️ Estimated effort to review: 3 🔵🔵🔵⚪⚪

🧪 No relevant tests

🔒 No security concerns identified

⚡ Key issues to review Hardcoded SHA256 The SHA256 hashes for Node.js and Rust are hardcoded in the flake file. This might lead to potential build failures if the packages at these hashes are removed or corrupted in the future. Consider a more dynamic approach to handle dependencies. Dependency Management The PR introduces new dependencies (fenix, nixpkgs-node-18) without removing or updating existing dependencies. Ensure that all dependencies are necessary and maintained to avoid security vulnerabilities and package conflicts.