Only calculate start window timestamp once in BaseTimeWindowPartition…

…sSubset.__contains__ (dagster-io#21792)

Summary:
speedscope suggests this might actually make a difference when there are
large numbers of time windows in the subset (for example, a */15 asset
or hourly asset going back to 2015)

Test Plan: BK

## Summary & Motivation

## How I Tested These Changes

python_modules/dagster/dagster/_core/definitions/time_window_partitions.py

@@ -1890,9 +1890,11 @@ def __contains__(self, partition_key: str) -> bool:
  TimeWindowPartitionsDefinition, self.partitions_def
  ).time_window_for_partition_key(partition_key)
 
+ time_window_start_timestamp = time_window.start.timestamp()
+
  return any(
- time_window.start.timestamp() >= included_time_window.start.timestamp()
- and time_window.start.timestamp() < included_time_window.end.timestamp()
+ time_window_start_timestamp >= included_time_window.start.timestamp()
+ and time_window_start_timestamp < included_time_window.end.timestamp()
  for included_time_window in self.included_time_windows
  )