Fix label permissions for Presubmit-BoringSSL-Verified

I had intended to only let the bot vote +1 on this label, and not
allow human users to vote +1 manually. But it seems this does not work,
as the CV PostAction is failing with "all votes failed":
https://luci-change-verifier.appspot.com/ui/run/boringssl/8848666313854-1-cd1eaded4dff834d

Change-Id: I6039491c044bfcfcfc5a40a950acb2ecdcfc0fe9
Reviewed-on: https://boringssl-review.googlesource.com/c/All-Projects/+/91367
Reviewed-by: Adam Langley <agl@google.com>