flake.lock: Update

Flake lock file updates:

• Updated input 'nixpkgs':
    'git+https://github.com/NixOS/nixpkgs?ref=nixos-unstable-small&rev=12d516726903aae3c89fe2b6b65a6880c7101c25&shallow=1' (2025-07-02)
  → 'git+https://github.com/NixOS/nixpkgs?ref=nixos-unstable-small&rev=6ec4d5f023c3c000cda569255a3486e8710c39bf&shallow=1' (2025-07-09)
• Updated input 'treefmt-nix':
    'github:numtide/treefmt-nix/ac8e6f32e11e9c7f153823abc3ab007f2a65d3e1?narHash=sha256-0IEdQB1nS%2BuViQw4k3VGUXntjkDp7aAlqcxdewb/hAc%3D' (2025-06-26)
  → 'github:numtide/treefmt-nix/c9d477b5d5bd7f26adddd3f96cfd6a904768d4f9?narHash=sha256-19m7P4O/Aw/6%2BCzncWMAJu89JaKeMh3aMle1CNQSIwM%3D' (2025-07-09)

Author: github-actions[bot]