flake.lock: Update (#1890)

Flake lock file updates:

• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/0e251e2' (2026-08-13)
  → 'github:NixOS/nixpkgs/2c423e0' (2026-08-22)
• Updated input 'rust-overlay':
    'github:oxalica/rust-overlay/ad8ebb5' (2026-08-15)
  → 'github:oxalica/rust-overlay/fd2ebb9' (2026-08-22)

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Raphael Amorim <rapha850@gmail.com>