scheduler.py: Move cache_size logs into folder

This prevents the cache_size.pid.log files from cluttering the root
log directory.

buildstream/_scheduler/scheduler.py

@@ -329,7 +329,7 @@ class Scheduler():
         self.schedule_jobs([job])
 
     def _check_cache_size_real(self):
-        job = CacheSizeJob(self, 'cache_size', 'cache_size',
+        job = CacheSizeJob(self, 'cache_size', 'cache_size/cache_size',
                            resources=[ResourceType.CACHE,
                                       ResourceType.PROCESS],
                            exclusive_resources=[ResourceType.CACHE],