cmd/releasebot: don't try to recreate the tag when it already exists
cmd/releasebot: don't try to recreate the tag when it already exists

Change-Id: I42865f871df540eeb98abd104a8c0560fdf34cd6
Reviewed-on: https://go-review.googlesource.com/c/154343
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/build,golang/build,golang/build,golang/build