CI: Run tests daily to detect incompatible django-stubs changes

[intgr]

I have made things!

Motivated by examples like #518, #508 and many others. It happens every few weeks that a change in django-stubs also requires followup changes in djangorestframework-stubs.

Currently, I only find out when someone opens a PR against DRF-stubs and then CI fails. Having scheduled CI would help me be more proactive about these.

Alternatively, we could also change django-stubs CI to automatically trigger workflow_dispatch in DRF-stubs repo on every merge to master. But that would have more moving parts. I think daily schedule is good enough for the time being.