ci: ✨ run tests on a nightly basis

[ericapisani]

Which problem is this pull request solving?

Adds a nightly cron job to run on the default/base (in this case main) branch.

Describe the solution you've chosen

Used the on.schedule configuration provided by Github Actions

Checklist

Please add a x inside each checkbox:

• I have read the contribution guidelines.
• I have added tests (we are enforcing 100% test coverage).
• I have added documentation in the README.md, the docs directory (if
  any) and the examples directory (if any).
• The status checks are successful (continuous integration). Those can be
  seen below.

[netlify]

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