minor GCP cost model fix (#5157)

Author: lcouzens

koku/masu/processor/tasks.py

@@ -883,9 +883,9 @@ def update_cost_model_costs(
         None
 
     """
-    # Override cost model start date to calculate costs for full month
+    # Override cost model start date str to calculate costs for full month
     LOG.info("overriding cost model start date to process full month")
-    start_date = DateHelper().month_start(start_date)
+    start_date = DateHelper().month_start(start_date).strftime("%Y-%m-%d")
     task_name = "masu.processor.tasks.update_cost_model_costs"
     cache_args = [schema_name, provider_uuid, start_date, end_date]
     if not synchronous: