flake.lock: Update

Flake lock file updates:

• Updated input 'neovim-nightly':
    'github:neovim/neovim/a8fc94a2fdd07edf0679d34f2deaacb25b3fa69f?dir=contrib' (2023-10-18)
  → 'github:neovim/neovim/594ff34581907fed4956d4dc93c033dfdd5f7a69?dir=contrib' (2023-10-19)
• Updated input 'nixpkgs':
    'github:nixos/nixpkgs/12bdeb01ff9e2d3917e6a44037ed7df6e6c3df9d' (2023-10-15)
  → 'github:nixos/nixpkgs/ff0a5a776b56e0ca32d47a4a47695452ec7f7d80' (2023-10-19)
• Updated input 'pre-commit-hooks':
    'github:cachix/pre-commit-hooks.nix/42e1b6095ef80a51f79595d9951eb38e91c4e6ca' (2023-10-09)
  → 'github:cachix/pre-commit-hooks.nix/8cc349bfd082da8782b989cad2158c9ad5bd70fd' (2023-10-19)