Purge persistent cache for perf test runs (#67697)

diff --git a/dev/devicelab/lib/tasks/perf_tests.dart b/dev/devicelab/lib/tasks/perf_tests.dart
index f8113b1..fa8aa57 100644
--- a/dev/devicelab/lib/tasks/perf_tests.dart
+++ b/dev/devicelab/lib/tasks/perf_tests.dart
@@ -746,6 +746,7 @@
         'run',
         '--verbose',
         '--verbose-system-logs',
+        '--purge-persistent-cache',
         '--profile',
         if (cacheSkSL) '--cache-sksl',
         '-d', _device.deviceId,