cmd/go: temporarily disable build JSON in test output

cmd/go: temporarily disable build JSON in test output

Unfortunately, this is tripping up the LUCI test output processor, so
we need to disable it until we can figure that out.

For #70402.
Updates #62067.

Cq-Include-Trybots: luci.golang.try:gotip-darwin-arm64_13,gotip-linux-amd64-longtest
Change-Id: I9ae722218e98b8060b8b4c46358f23381ac8537a
Reviewed-on: https://go-review.googlesource.com/c/go/+/628955
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>