Run CI weekly

Is your feature request related to a problem? Please describe.

Now that VirtualShipcode v1 has been released and the development of the code will go slower, we may not realise if there are breaking changes in some of the dependencies. Especially the API of Parcels could for example still change a bit.

Describe the solution you'd like
It would be good to run (weekly?) GH Actions and report on failure, so that I can check what's going on