x/build/cmd/gopherbot: fix Gerrit reviewer email addresses

I have three main email addresses (gmail.com, golang.org, google.com), with three corresponding Chrome identities/Google accounts.

So do many others on the Go team. (Or at least two)

And because I've clicked links to Gerrit reviews while signed into each of the three, Gerrit has helpfully signed me in & made an account for me.

Which then means my name shows up in the autocomplete list for "bradfitz" in three ways.

It's distracting when I get reviews to the wrong email account.

This just bit @ianlancetaylor too.

I realized we could just have a mapping table and make gopherbot fix the reviewers if it sees a wrong one added by mistake.

/cc @dmitshur @andybons @bcmills