flake.lock: Update (#1899)

Flake lock file updates:

• Updated input 'nixpkgs':
    'github:nixos/nixpkgs/600faf99b58477c130ea8e95865974acbfd86950?narHash=sha256-CN4he2Ed3WYU4uaYVjVCeH23xB8u3g0H34%2BWCljaCjI%3D' (2025-07-20)
  → 'github:nixos/nixpkgs/b569c6cb485e91b17ba09258743126bd32ff2637?narHash=sha256-HheX0gl4%2BoFqp/jMQh7RPVYJdUe41nH%2BMlFGjMrOVSM%3D' (2025-07-27)

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Author: github-actions[bot]