flake.lock: Update (#1937)

Flake lock file updates:

• Updated input 'nixpkgs':
    'github:nixos/nixpkgs/b536f7a97c062729a9be3e32d1b49301cdd94404?narHash=sha256-9XRtmLGdvRfk/HNIDwRkQHNzMBOXwGDNHWWD4YaoU0o%3D' (2025-08-31)
  → 'github:nixos/nixpkgs/38c4f0e41050b86b581ef230dede92cd1ae99400?narHash=sha256-qAvbwMEoqvS%2BiI2MdBMbNrsl5IBIMVNcIs5BEsyMcPY%3D' (2025-09-07)

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

Author: github-actions[bot]