flake.lock: Update (#395)

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/22c3f2cf41a0e70184334a958e6b124fb0ce3e01?narHash=sha256-Qn3nPMSopRQJgmvHzVqPcE3I03zJyl8cSbgnnltfFDY%3D' (2024-12-07)
  → 'github:nixos/nixpkgs/a73246e2eef4c6ed172979932bc80e1404ba2d56?narHash=sha256-463SNPWmz46iLzJKRzO3Q2b0Aurff3U1n0nYItxq7jU%3D' (2024-12-09)
```

### Running GitHub Actions on this PR

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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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