Call evaluate_pre() instead of evaluate() on Maybe decider

e19142c introduced evaluate_pre to
perform context unrolling before to call the semi-public evaluate().

The Maybe decider was not updated at that time, but its context need to
be unrolled.

factory/declarations.py

@@ -536,7 +536,7 @@ def evaluate_post(self, instance, step, overrides):
             return target
 
     def evaluate_pre(self, instance, step, overrides):
-        choice = self.decider.evaluate(instance=instance, step=step, extra={})
+        choice = self.decider.evaluate_pre(instance=instance, step=step, overrides={})
         target = self.yes if choice else self.no
         # The value can't be POST_INSTANTIATION, checked in __init__;
         # evaluate it as `evaluate_pre`