Disable prebuilt uploading for external toolchain bots.
Disable prebuilt uploading for external toolchain bots.

The toolchain bots shouldn't upload prebuilts.

BUG=none
TEST=./cbuildbot_config_unittest.py

Change-Id: Ibf91fce2cb98486a5defbbef0ebf351688043072
Reviewed-on: http://gerrit.chromium.org/gerrit/4357
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Tested-by: Raymes Khoury <348ad72dc039a127ca70ef38e5de4eebe052b884@chromium.org>

chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,coreos/chromite,zhang0137/chromite,chadversary/chromiumos.chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,zhang0137/chromite