Default to starting with a fresh chroot on remote trybot runs.

BUG=chromium-os:27122
TEST=Remote tryjob.

Change-Id: I2f810aaa588b4f2faf9737f0a2f54b6fc2d023a2
Reviewed-on: https://gerrit.chromium.org/gerrit/18147
Reviewed-by: David James <davidjames@chromium.org>
Commit-Ready: Ryan Cui <rcui@chromium.org>
Tested-by: Ryan Cui <rcui@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index 1a6367e..5f710ba 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -990,6 +990,7 @@
                                      cros_lib.Timeout, options.timeout):
           if not options.buildbot:
             build_config = cbuildbot_config.OverrideConfigForTrybot(
-                build_config)
+                build_config,
+                options.remote_trybot)
 
           _RunBuildStagesWrapper(options, build_config)