Fix issue where we were calling the partition function n times, where…

… n is the number of partitions returned by the function (#7539)

Summary:
The old logic is not efficient when this function does non-trivial work.

Test Plan: Existing BK, run a schedule that returns multiple partitions and verify with logs that it is no longer called in a loop

python_modules/dagster/dagster/core/definitions/partition.py

@@ -594,10 +594,12 @@ def _execution_fn(context):
                 yield SkipReason("Partition selector returned an empty list of partitions.")
                 return
 
+            partition_names = self.get_partition_names(context.scheduled_execution_time)
+
             missing_partition_names = [
                 partition.name
                 for partition in selected_partitions
-                if partition.name not in self.get_partition_names(context.scheduled_execution_time)
+                if partition.name not in partition_names
             ]
 
             if missing_partition_names: