Update flake.lock

[klarkc]

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

Flake lock file updates:

• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/46688f8eb5cd6f1298d873d4d2b9cf245e09e88e' (2023-09-15)
  → 'github:NixOS/nixpkgs/e12483116b3b51a185a33a272bf351e357ba9a99' (2023-09-21)

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