flake.lock: Update (#336)

Automated changes by the
[update-flake-lock](https://github.com/DeterminateSystems/update-flake-lock)
GitHub Action.

```
Flake lock file updates:

• Updated input 'nixpkgs':
    'github:nixos/nixpkgs/2768c7d042a37de65bb1b5b3268fc987e534c49d?narHash=sha256-AlcmCXJZPIlO5dmFzV3V2XF6x/OpNWUV8Y/FMPGd8Z4%3D' (2024-10-23)
  → 'github:nixos/nixpkgs/18536bf04cd71abd345f9579158841376fdd0c5a?narHash=sha256-RP%2BOQ6koQQLX5nw0NmcDrzvGL8HDLnyXt/jHhL1jwjM%3D' (2024-10-25)
```

### 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:

```sh
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
```

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