main.star: add NEXTUP_GO branch for Go 1.27 release candidate Change-Id: I5f373c3359c3e50f94ce23f228f8db89decc9a39 Reviewed-on: https://go-review.googlesource.com/c/build/+/789900 Reviewed-by: Mark Freeman <markfreeman@google.com> LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>