Update nix flakes

[CopeFiend]

Description

Update flake.nix in source and template to fix breaking changes with Nix upstream, plus rust-src path for LSP support.

Details

• Updated flake.nix:
  • Moved build-time tools (pkg-config, rust-bin) to nativeBuildInputs
  • Added runtime libraries.
  • Add RUST_SRC_PATH to enable IDE/LSP support
• Generated new flake.lock

Review Guidance

• System: x86_64.
• Nix branch: nixos-unstable
• flake.lock generated using nix develop command.

Checklist

• My code follows the project's code style guidelines
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly
• My changes don't generate new warnings or errors
• No unsafe code introduced (or if introduced, thoroughly justified and documented)

[CopeFiend]

Question: Is flake.lock even useful given how often nixos-unstable updates?

Right now, the provided flake.lock will produce deprecation warnings, and eventually the updated one will too.

A solution would be to select a specific Nix branch, but that would mean losing access to latest changes. The alternative is to push a new flake.lock to master every few weeks. Neither seems too attractive.

[mrDIMAS]

I don't have any experience with nixOS, all of these existing configs were added by other contributors.