Fix constraint cloning error (#6275)
Fix constraint cloning error (#6275)

## What is the goal of this PR?
Fix an error in a demo that occurs when a labeled variable is shared across concludables and retrievables.

## What are the changes implemented in this PR?
* Let the constraint cloner clone labeled variables label constraint multiple times
graknlabs/grakn,graknlabs/grakn,graknlabs/grakn,graknlabs/grakn