patch to fix #5145

underlying issue is that model updates for multi-objective and single objective solving are too brittle to serve its use cases among different plugins.
For maxlex, the last model is always the best and it doesn't use multiple objectives.

src/opt/opt_solver.cpp

@@ -191,6 +191,8 @@ namespace opt {
         r = adjust_result(r);
         if (r == l_true) {
             m_context.get_model(m_last_model);
+            if (m_models.size() == 1)
+                m_models.set(0, m_last_model.get());
         }
         m_first = false;
         if (dump_benchmarks()) {