ImageService/Create: Output suffix for factory images

Release builders doing official builds end up with image build
directories that have the same name for every call to build_image.
Add a suffix to the release builder call to force unique names.

BUG=None
TEST=run_tests, manual

Change-Id: Ifc778a107a70040259e837c24a371e9b73083f39
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3149334
Tested-by: Alex Klein <saklein@chromium.org>
Reviewed-by: George Engelbrecht <engeg@google.com>
Commit-Queue: Alex Klein <saklein@chromium.org>
diff --git a/api/controller/image.py b/api/controller/image.py
index ab66f11..257c870 100644
--- a/api/controller/image.py
+++ b/api/controller/image.py
@@ -215,6 +215,7 @@
   factory_build_config = copy.copy(build_config)
   build_config.symlink = LOCATION_CORE
   factory_build_config.symlink = LOCATION_FACTORY
+  factory_build_config.output_dir_suffix = LOCATION_FACTORY
 
   # Try building the core and factory images.
   # Sorted isn't really necessary here, but it's much easier to test.