chore: update flake.lock

[github-actions]

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

Flake lock file updates:

• Updated input 'nixpkgs':
    'github:nixos/nixpkgs/4cdad15f34e6321a2f789b99d42815b9142ac2ba' (2023-08-19)
  → 'github:nixos/nixpkgs/475d5ae2c4cb87b904545bdb547af05681198fcc' (2023-08-20)

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