Python: Prevent bad joins in TypeBackTracker

Perhaps unsurprisingly, the join orderer was eager and willing to find
the wrong join order in this predicate as well. Applying a similar
fix to the one used in `TypeTracker::step` fixes the problem.

Author: tausbn

python/ql/src/semmle/python/dataflow/new/TypeTracker.qll

@@ -411,8 +411,8 @@ class TypeBackTracker extends TTypeBackTracker {
   pragma[inline]
   TypeBackTracker step(LocalSourceNode nodeFrom, LocalSourceNode nodeTo) {
     exists(StepSummary summary |
-      StepSummary::step(nodeFrom, nodeTo, summary) and
-      this = result.prepend(summary)
+      StepSummary::step(pragma[only_bind_out](nodeFrom), nodeTo, pragma[only_bind_into](summary)) and
+      this = result.prepend(pragma[only_bind_into](summary))
     )
   }