Tweak OR-Tools CP-SAT solver parameters

Following suggestions in google/or-tools#4166

slothy/core/core.py

@@ -3130,6 +3130,9 @@ def _init_external_model_and_solver(self):
         self._model.cp_solver = cp_model.CpSolver()
         self._model.cp_solver.random_seed = self.config.solver_random_seed
 
+        self._model.cp_solver.parameters.search_bracnhing = cp_model.FIXED_SEARCH
+        self._model.cp_solver.parameters.num_workers = 1
+
         # There is a bug in OR-Tools, https://github.com/google/or-tools/issues/3483,
         # that causes models to be incorrectly classes as INFEASIBLE at times.
         # The following turns of the buggy parts of the code: