[chore] flake.lock: update inputs

Flake lock file updates:

• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/394571358ce82dff7411395829aa6a3aad45b907' (2024-12-16)
  → 'github:NixOS/nixpkgs/b47fd6fa00c6afca88b8ee46cfdb00e104f50bca' (2024-12-19)
• Updated input 'nixvim':
    'github:nix-community/nixvim/37608b462772e35220e02bfbd9045d0946564436' (2024-12-19)
  → 'github:nix-community/nixvim/354fc0f288e045594e2e30da7ff95189d3545a9a' (2024-12-20)
• Updated input 'treefmt-nix':
    'github:numtide/treefmt-nix/76159fc74eeac0599c3618e3601ac2b980a29263' (2024-12-18)
  → 'github:numtide/treefmt-nix/65712f5af67234dad91a5a4baee986a8b62dbf8f' (2024-12-20)