create_experiment_workspace doesn't create singleton collections

The create_experiment_workspace task (and hence the corresponding workflow) doesn't create the usual singleton collections in the new workspace. I guess they're mostly inherited from the parent, but I noticed this because I was working on some manual setup for #1093 and found that the experiment workspace didn't have a debian:archive collection.

I think create_experiment_workspace should create these collections in much the same way that debusine-admin workspace define does. It could have a task data parameter defaulting to True to allow disabling this behaviour, although off the top of my head I don't have a use case for disabling it.