GPMixin: Fix keep_soft_constraints with ensembles

We rely on the map_path_expression() call working for all ensemble members. For performance reasons, map_path_expression() reuses variables from transcribe(), see f94499ca. Therefore, we cannot call map_path_expression() in the constraints() call of any ensemble member later than currently being processed. So instead of adding our overall constraint to the first ensemble member, we have to add it to the last.