Still another try at fixing scanjoin_target insertion into parallel plans.

Still another try at fixing scanjoin_target insertion into parallel plans.

The previous code neglected the fact that the scanjoin_target might
carry sortgroupref labelings that we need to absorb.  Instead, do
create_projection_path() unconditionally, and tweak the path's cost
estimate after the fact.  (I'm now convinced that we ought to refactor
the way we account for sometimes not needing a separate projection step,
but right now is not the time for that sort of cleanup.)

Problem identified by Amit Kapila, patch by me.