flake.lock: Update (#1908)

Flake lock file updates:

• Updated input 'nixpkgs':
    'github:nixos/nixpkgs/b569c6cb485e91b17ba09258743126bd32ff2637?narHash=sha256-HheX0gl4%2BoFqp/jMQh7RPVYJdUe41nH%2BMlFGjMrOVSM%3D' (2025-07-27)
  → 'github:nixos/nixpkgs/54b5c7b431c4ca500e6f1e139c66e0f91c61a15c?narHash=sha256-bus8ai5MP3wqVp7qSui3bKNht8bDSy9koDH1ID0BH/E%3D' (2025-08-03)

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]