[chore] flake.lock: update inputs

Flake lock file updates:

```
• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/8c50662509100d53229d4be607f1a3a31157fa12' (2024-07-27)
  → 'github:NixOS/nixpkgs/cf05eeada35e122770c5c14add958790fcfcbef5' (2024-07-30)
• Updated input 'nixvim':
    'github:nix-community/nixvim/2415edc0cb749bf81c9b142138c2bb705514f6cc' (2024-07-29)
  → 'github:nix-community/nixvim/7c39d77b9f1fbcbd8f2a575c4f2948dd54efc5c1' (2024-08-01)
• Updated input 'nixvim/treefmt-nix':
    'github:numtide/treefmt-nix/8db8970be1fb8be9c845af7ebec53b699fe7e009' (2024-07-23)
  → 'github:numtide/treefmt-nix/768acdb06968e53aa1ee8de207fd955335c754b7' (2024-07-30)
• Updated input 'unstable':
    'github:NixOS/nixpkgs/b73c2221a46c13557b1b3be9c2070cc42cf01eb3' (2024-07-27)
  → 'github:NixOS/nixpkgs/9f918d616c5321ad374ae6cb5ea89c9e04bf3e58' (2024-07-31)
```