flake: update lockfile

[github-actions]

Automated changes by the update-flake-lock GitHub Action.

Flake lock file updates:

• Updated input 'nixpkgs':
    'https://releases.nixos.org/nixos/26.05/nixos-26.05.8954.a5cc6f2c37bf/nixexprs.tar.xz?narHash=sha256-z3ACU3NTAstjK%2BJNjW0zmIH2Q0GYo2pgGfJddesv7Xo%3D' (2026-09-03)
  → 'https://releases.nixos.org/nixos/26.05/nixos-26.05.9227.c25784012c99/nixexprs.tar.xz?narHash=sha256-aVla%2BxR1lotA4iEMo6ltSr5oKVvWxBZ8Gc9KjbwxJq8%3D' (2026-09-06)

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