Run screenshot tests daily, instead of on each commit

[andybalaam]

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This change is marked as an internal change (Task), so will not be included in the changelog.

[t3chguy]

This heavily duplicates cypress.yaml - please make use of composite actions and/or reusable workflows otherwise when updating things we are likely to diverge them

[andybalaam]

Thanks, I was about to ask you about this :-)

[andybalaam]

I ran this on my fork overnight and it kicked off the Cypress tests as expected. Those tests failed in the same way as the normal Cypress tests, so there is a decent chance this is doing what I hope it will do.

The part I am not sure about is github.event.workflow_run.event == 'schedule'. I can't find any docs that help me know for sure whether this will work. Maybe I could try a mini test of this part on a fork, or maybe we just merge it here and see whether it works...