chore: update flake.lock

[mrcjkb]

Automated changes by the update-flake-lock GitHub Action.

Flake lock file updates:

• Updated input 'neovim-nightly':
    'github:neovim/neovim/25cfe3fd432d77689446fe5a0bb972479298387c?dir=contrib' (2023-10-23)
  → 'github:neovim/neovim/684e93054b82c6b5b215db7d3ecbad803eb81f0e?dir=contrib' (2023-10-25)

Running GitHub Actions on this PR

GitHub Actions will not run workflows on pull requests which are opened by a GitHub Action.

To run GitHub Actions workflows on this PR, run:

git branch -D update_flake_lock_action
git fetch origin
git checkout update_flake_lock_action
git commit --amend --no-edit
git push origin update_flake_lock_action --force