project.config: grant LUCI trybot vote on internal branches too

Mostly to test that CL 522179 works, but it'll also be useful in the
rare case someone needs to make a cherry-pick for a vendored package.

Change-Id: I40c0154439011afcdd0a50401e75c2fca46cbe2f
Reviewed-on: https://go-review.googlesource.com/c/All-Projects/+/522535
Reviewed-by: Heschi Kreinick <heschi@google.com>