chore: daily flake.lock update (#5707)

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/dfinity/motoko/blob/master/.github/workflows/update-flake-lock.yml)
and run in:
https://github.com/dfinity/motoko/actions/runs/19979829074/attempts/1:

```
Flake lock file updates:

• Updated input 'motoko-core-src':
    'github:dfinity/motoko-core/d980c15' (2025-12-04)
  → 'github:dfinity/motoko-core/cae618a' (2025-12-05)
```

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

Author: pr-automation-bot-public[bot]