Update flake.lock (#346)

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/dc14ed91132ee3a26255d01d8fd0c1f5bff27b2f?narHash=sha256-adRZhFpBTnHiK3XIELA3IBaApz70HwCYfv7xNrHjebA%3D' (2024-07-23)
  → 'github:nixos/nixpkgs/9f918d616c5321ad374ae6cb5ea89c9e04bf3e58?narHash=sha256-/DJBI6trCeVnasdjUo9pbnodCLZcFqnVZiLUfqLH4jA%3D' (2024-07-31)
```

### 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>

Author: simonrw