cubicweb #6694406 don't rely on equality on rql.nodes.VariableRef [validation pending]

We should explicitly compare the referenced variable when that's what we want to do, not rely on the comparison operators doing that. See https://www.logilab.org/1190458

prioritynormal
typebug
done in3.22.0
load1.000
load left0.000
closed by#e3e4a8c45695 [server/rql2sql] use VariableRef.is_equivalent explicitly instead of relying on __eq__
patch[server/rql2sql] use VariableRef.is_equivalent explicitly instead of relying on __eq__ [applied]