cubicweb-collaboration #4383924 build_collaboration_workflow should not attempt to add the workflow if there's already one [validation pending]

e.g. if one calls the method twice in different migration script

prioritynormal
typebug
done in1.0.1
load0.100
load left0.000
closed by#f7e85e1b4ceb [config] Do not build the collaboration workflow if there exists already one
patch[config] Do not build the collaboration workflow if there exists already one [applied][migration] Fix buggy 1.0.0 migration [applied]