language/environment: only call prepare automatically if it exists

artiq/language/environment.py

@@ -380,8 +380,9 @@ class EnvExperiment(Experiment, HasEnvironment):
     Most experiment should derive from this class."""
     def prepare(self):
         """The default prepare method calls prepare for all children, in the
-        order of instantiation."""
+        order of instantiation, if the child has a prepare method."""
         for child in self.children:
+            if hasattr(child, "prepare"):
                 child.prepare()