x/build/cmd/gopherbot: add author to issue notification comments

gopherbot generates the "CL mentions this issue" issue comments:

https://github.com/golang/build/blob/4639cd3919e4243a672a11a56543004bf1778326/cmd/gopherbot/gopherbot.go#L811

It currently includes a link to the CL and the CL title. It'd be nice if it also included the CL author's name or handle. Maybe: "Change `<link>` by `<author>` mentions this issue: `<title>`".
