Restore gobot's permission to cast TryBot-Result

gobot was using its membership of trybot-result-changers to do that,
but that group was switched to casting the vote on behalf of in
I90c09cba2e157152de6946f592f9833537841827

Change-Id: I94df0d7c546ecef94f783ff1f175374773ab43cc
Reviewed-on: https://go-review.googlesource.com/116764
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>