[autotest] Default build to CrOS build in create_suite_job

The build meant to be default to CrOS build. Set it to test_source_build
introduced a bug when test_source_build is a firmware build.

BUG=None
TEST=Local

Change-Id: Ibb81820cd75b0a9fb6544092297f96fc9c04a744
Reviewed-on: https://chromium-review.googlesource.com/281925
Reviewed-by: Simran Basi <sbasi@chromium.org>
Reviewed-by: Dan Shi <dshi@chromium.org>
Commit-Queue: Dan Shi <dshi@chromium.org>
Trybot-Ready: Dan Shi <dshi@chromium.org>
Tested-by: Dan Shi <dshi@chromium.org>
diff --git a/frontend/afe/site_rpc_interface.py b/frontend/afe/site_rpc_interface.py
index e6d7849..7127265 100644
--- a/frontend/afe/site_rpc_interface.py
+++ b/frontend/afe/site_rpc_interface.py
@@ -218,7 +218,7 @@
     # R45 falls out of stable channel.
     # Prepend build and board to the control file.
     inject_dict = {'board': board,
-                   'build': test_source_build,
+                   'build': builds[provision.CROS_VERSION_PREFIX],
                    'builds': builds,
                    'check_hosts': check_hosts,
                    'pool': pool,