Use git-cache for GCE builders.

Fix HostIsCIBuilder so --cache-dir can be applied to GCE builders.

BUG=chromium:637101
TEST=unit_test

Change-Id: I46d522251335f4e236cadd4fba2ed8de225adf8c
Reviewed-on: https://chromium-review.googlesource.com/368352
Commit-Ready: Ningning Xia <nxia@chromium.org>
Tested-by: Ningning Xia <nxia@chromium.org>
Reviewed-by: David James <davidjames@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index 6ec5e9d..ec20ebc 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -1136,6 +1136,11 @@
         and not options.resume and not options.local):
     cros_build_lib.Die('Please use --remote or --local to run trybots')
 
+  elif options.buildbot and not options.debug:
+    if not cros_build_lib.HostIsCIBuilder():
+      # Cannot run --buildbot if both --debug and --remote aren't specified.
+      cros_build_lib.Die('This host isn\'t a continuous-integration builder.')
+
   # Only one config arg is allowed in this mode, which was confirmed earlier.
   bot_id = args[-1]
   build_config = site_config[bot_id]