flake.lock: Update (#1417)

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/7bb2ccd8cdc44c91edba16c48d2c8f331fb3d856' (2024-04-25)
  → 'github:NixOS/nixpkgs/63c3a29ca82437c87573e4c6919b09a24ea61b0f' (2024-05-02)
```

### 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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