CI: try out cargo hack

[Mirko-von-Leipzig]

Test out cargo hack check --workspace (or some variation of the incantation) in CI.

This will prevent incorrect dependency feature definitions which are obscured due to dependency feature unification that cargo performs by default. Some further context in this [comment].

This addition is just a nice-to-have; so if this works poorly, takes long (don't forget to add ci caching) then we shouldn't include it.