[server/rql2sql] use VariableRef.is_equivalent explicitly instead of relying on __eq__
What we want to compare is the variable referenced by the VariableRef,
we should make that explicit.
Closes #6694406
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html