main.star: add presubmits and x repos to freebsd-amd64_15.0

This builder is nearly working, so we need to be able to trigger tests.

Updates golang/go#77411.

Change-Id: I6a6a636ca177844298b2c4bd856848a20224c143
Reviewed-on: https://go-review.googlesource.com/c/build/+/747041
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>