chore: daily flake.lock update (#6052)

Automated changes by the
[update-flake-lock](https://github.com/DeterminateSystems/update-flake-lock)
GitHub Action.
as defined in
[.github/workflows/update-flake-lock.yml](https://github.com/caffeinelabs/motoko/blob/master/.github/workflows/update-flake-lock.yml)
and run in:
https://github.com/caffeinelabs/motoko/actions/runs/24866392912/attempts/1:

```
Flake lock file updates:

• Updated input 'motoko-core-src':
    'github:caffeinelabs/motoko-core/1b6e499' (2026-04-21)
  → 'github:caffeinelabs/motoko-core/1577a86' (2026-04-23)
```

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: caffeine-ci-generic-rw[bot]