lib: sysroot_lib: Enable use of CQ binhosts with flag

This is a reland of enabling CQ prebuilts but it enables it only
with the flag.

The feature was once enabled but was reverted by crrev.com/c/4540304
and crrev.com/c/4535496, so that this CL makes the logic work only
when the flag is set.

Currently the flag is disabled for all builders, so that this CL
should not change any behavior.

BUG=b:283060147
TEST=./run_tests lib/sysroot_lib_unittest.py

Cq-Depend: chromium:4555076
Change-Id: I61d0c9feddb15d78c80895cb2b31d42f8ff81bf1
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4542884
Reviewed-by: Cindy Lin <xcl@google.com>
Commit-Queue: Yoshiki Iguchi <yoshiki@chromium.org>
Tested-by: Yoshiki Iguchi <yoshiki@chromium.org>
diff --git a/api/controller/sysroot.py b/api/controller/sysroot.py
index 0ab7e72..ecb3287 100644
--- a/api/controller/sysroot.py
+++ b/api/controller/sysroot.py
@@ -174,6 +174,7 @@
     """Create or replace a sysroot."""
     update_chroot = not input_proto.flags.chroot_current
     replace_sysroot = input_proto.flags.replace
+    use_cq_prebuilts = input_proto.flags.use_cq_prebuilts
 
     build_target = controller_util.ParseBuildTarget(
         input_proto.build_target, input_proto.profile
@@ -186,6 +187,7 @@
         force=replace_sysroot,
         upgrade_chroot=update_chroot,
         package_indexes=package_indexes,
+        use_cq_prebuilts=use_cq_prebuilts,
         backtrack=DEFAULT_BACKTRACK,
     )