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-26.05.8409.f4f698677b11/nixexprs.tar.xz?narHash=sha256-tbu/95O8ZRmCj0kfSiDvz3YT7GSnD0fO6cxLRVYtXnU%3D' (2026-08-25)
  → 'https://releases.nixos.org/nixos/26.05/nixos-26.05.8538.d57af924f160/nixexprs.tar.xz?narHash=sha256-pbm8BrmtI4lOXt2syJ3FP12LjybYuiTdUO0GQrbQMk0%3D' (2026-08-27)

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