flake.lock: Update

Flake lock file updates:

• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/8b5ab8341e33322e5b66fb46ce23d724050f6606' (2023-09-19)
  → 'github:NixOS/nixpkgs/e12483116b3b51a185a33a272bf351e357ba9a99' (2023-09-21)
• Updated input 'sops-nix':
    'github:Mic92/sops-nix/ea208e55f8742fdcc0986b256bdfa8986f5e4415' (2023-09-12)
  → 'github:Mic92/sops-nix/2f375ed8702b0d8ee2430885059d5e7975e38f78' (2023-09-21)
• Updated input 'sops-nix/nixpkgs-stable':
    'github:NixOS/nixpkgs/5601118d39ca9105f8e7b39d4c221d3388c0419d' (2023-09-02)
  → 'github:NixOS/nixpkgs/596611941a74be176b98aeba9328aa9d01b8b322' (2023-09-16)
• Updated input 'treefmt-nix':
    'github:numtide/treefmt-nix/7a49c388d7a6b63bb551b1ddedfa4efab8f400d8' (2023-09-12)
  → 'github:numtide/treefmt-nix/e951529be2e7c669487de78f5aef8597bbae5fca' (2023-09-21)