flake.lock: Update

Flake lock file updates:

• Updated input 'nixpkgs':
    'git+https://github.com/NixOS/nixpkgs?ref=nixos-unstable-small&rev=6a2957c7978b189202e03721aab901c0a9dc1e1a&shallow=1' (2025-04-26)
  → 'git+https://github.com/NixOS/nixpkgs?ref=nixos-unstable-small&rev=a4b6cee9ab63822643e108f67da016aa800ea099&shallow=1' (2025-05-03)
• Updated input 'treefmt-nix':
    'github:numtide/treefmt-nix/8d404a69efe76146368885110f29a2ca3700bee6?narHash=sha256-aRmUh0AMwcbdjJHnytg1e5h5ECcaWtIFQa6d9gI85AI%3D' (2025-04-18)
  → 'github:numtide/treefmt-nix/29ec5026372e0dec56f890e50dbe4f45930320fd?narHash=sha256-4h3s1L/kKqt3gMDcVfN8/4v2jqHrgLIe4qok4ApH5x4%3D' (2025-05-02)

Author: github-actions[bot]