talks: fix try-bots adding build OMITs everywhere

Fixes golang/go#19154.

Change-Id: I1deb2b762cada048945601db2d1a737b5c0be3e7
Reviewed-on: https://go-review.googlesource.com/37210
Run-TryBot: Francesc Campoy Flores <campoy@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>