[autotest] Keep build arg for suite control files that need it.

Some suite control files, e.g., control.au, still uses variable `suite`
to stage image inside the control file. Therefore, the variable still
needs to be injected into the control file.

BUG=chromium:496782
TEST=push to prod test

Change-Id: I73f80f251d6b8ce7307d6fd1178b2ed786e3bd1b
Reviewed-on: https://chromium-review.googlesource.com/337691
Commit-Ready: Dan Shi <dshi@chromium.org>
Tested-by: Dan Shi <dshi@google.com>
Reviewed-by: Richard Barnette <jrbarnette@chromium.org>
diff --git a/frontend/afe/site_rpc_interface.py b/frontend/afe/site_rpc_interface.py
index 1cdeb10..6de782c 100644
--- a/frontend/afe/site_rpc_interface.py
+++ b/frontend/afe/site_rpc_interface.py
@@ -258,6 +258,9 @@
 
     # Prepend builds and board to the control file.
     inject_dict = {'board': board,
+                   # `build` is needed for suites like AU to stage image inside
+                   # suite control file.
+                   'build': test_source_build,
                    'builds': builds,
                    'check_hosts': check_hosts,
                    'pool': pool,