flake.lock: Update (#1433)

Flake lock file updates:

• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/88d3861' (2026-01-21)
  → 'github:NixOS/nixpkgs/bfc1b8a' (2026-01-26)
• Updated input 'rust-overlay':
    'github:oxalica/rust-overlay/22da29e' (2026-01-24)
  → 'github:oxalica/rust-overlay/a1d32c9' (2026-01-31)

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

Author: github-actions[bot]