Add a sleep in order to allow provenance to fully upload

Sometimes the provenance isn't getting uploaded in time for BCID validation, so do a similar strategy as dart for validating provenance. Example: https://ci.chromium.org/p/dart-internal/builders/flutter/Mac%20Production%20Engine%20Drone/5843?

Change-Id: I3ffad3c80979640f4f2580ab83ce95f7c0fd5f5e
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/49060
Commit-Queue: Drew Roen <drewroen@google.com>
Reviewed-by: Jesse Seales <jseales@google.com>
(cherry picked from commit e695b9954ec642c0a249f2ff51d63fd04ee018ba)
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/50360
diff --git a/recipes/engine_v2/builder.py b/recipes/engine_v2/builder.py
index 79fbe0d..417c028 100644
--- a/recipes/engine_v2/builder.py
+++ b/recipes/engine_v2/builder.py
@@ -57,6 +57,7 @@
     'recipe_engine/properties',
     'recipe_engine/raw_io',
     'recipe_engine/step',
+    'recipe_engine/time',
 ]
 
 PROPERTIES = InputProperties
@@ -209,6 +210,8 @@
     for archive_config in archives:
       outputs[archive_config['name']] = Archive(api, checkout, archive_config)
     api.flutter_bcid.report_stage('upload-complete')
+    # Allow time for the provenance to upload so it can be validated
+    api.time.sleep(60)
     for archive_config in archives:
       if api.flutter_bcid.is_official_build():
         # TODO(drewroengoogle): Remove try-except block to make the verification