flake.lock: Update (#98)

Flake lock file updates:

• Updated input 'neovim-nightly':
    'github:neovim/neovim/7e9407490c51da5e677e5aea5eb48200600effbc?dir=contrib' (2023-10-06)
  → 'github:neovim/neovim/6823fdb20b1330142506410a62474166f646bf13?dir=contrib' (2023-10-07)
• Updated input 'nixpkgs':
    'github:nixos/nixpkgs/a2eb207f45e4a14a1e3019d9e3863d1e208e2295' (2023-10-06)
  → 'github:nixos/nixpkgs/945559664c1dc5836173ee12896ba421d9b37181' (2023-10-07)

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