Update flake.lock (#354)

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/9f918d616c5321ad374ae6cb5ea89c9e04bf3e58?narHash=sha256-/DJBI6trCeVnasdjUo9pbnodCLZcFqnVZiLUfqLH4jA%3D' (2024-07-31)
  → 'github:nixos/nixpkgs/cb9a96f23c491c081b38eab96d22fa958043c9fa?narHash=sha256-IAoYyYnED7P8zrBFMnmp7ydaJfwTnwcnqxUElC1I26Y%3D' (2024-08-04)
```

### 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
```

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