main.star: add darwin-amd64_15 builder

For golang/go#77997.

Change-Id: Icb32173e0e848cc37a6c153fe96ecb8076eba7a4
Reviewed-on: https://go-review.googlesource.com/c/build/+/752460
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>