artifacts: Add latest/ to image dir.

TEST=./run_tests
BUG=chromium:949566

Change-Id: I86b78b04da9a7a8f47dffa74179495157fad1e44
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1559153
Tested-by: Evan Hernandez <evanhernandez@chromium.org>
Reviewed-by: Jason Clinton <jclinton@chromium.org>
Reviewed-by: Sean Abraham <seanabraham@chromium.org>
diff --git a/api/controller/artifacts.py b/api/controller/artifacts.py
index 7d68206..8f0d829 100644
--- a/api/controller/artifacts.py
+++ b/api/controller/artifacts.py
@@ -27,7 +27,7 @@
   build_root = constants.SOURCE_ROOT
 
   # Use the first available image to create the update payload.
-  img_root = os.path.join(build_root, 'src/build/images', target)
+  img_root = os.path.join(build_root, 'src/build/images', target, 'latest')
   img_types = [
       constants.IMAGE_TYPE_TEST, constants.IMAGE_TYPE_DEV,
       constants.IMAGE_TYPE_BASE