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>
6 files changed