Schedule CI to run once per week so that cache doesn't get lost

add
```  
  schedule:
    # additionally run once per week (At 00:00 on Sunday) to maintain cache
    - cron: '0 0 * * 0'
```

to `on:` in github actions workflow.

Inspired by https://github.com/kowainik/stan/blob/main/.github/workflows/ci.yml .