flake.lock: Update

Flake lock file updates:

• Updated input 'flake-utils':
    'github:numtide/flake-utils/f9e7cf818399d17d347f847525c5a5a8032e4e44' (2023-08-23)
  → 'github:numtide/flake-utils/ff7b65b44d01cf9ba6a71320833626af21126384' (2023-09-12)
• Updated input 'nickel':
    'github:tweag/nickel/104881b56e58a34136fd5ec4fad5e4b158b616b9' (2023-09-14)
  → 'github:tweag/nickel/cf4c2a44b76100da1170f3a6e89a52be9cb44f93' (2023-09-15)
• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/db9208ab987cdeeedf78ad9b4cf3c55f5ebd269b' (2023-09-08)
  → 'github:NixOS/nixpkgs/f2ea252d23ebc9a5336bf6a61e0644921f64e67c' (2023-09-14)