cmd/go: mark gocacheverify godebug as undocumented

cmd/go: mark gocacheverify godebug as undocumented

This avoids the panic for godebugs without documentation. Perhaps we
eventually want to remove this godebug.

Change-Id: I032cdded0f3ccf6681821286e2cb6f0b90a318c3
Reviewed-on: https://go-review.googlesource.com/c/go/+/485756
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Bypass: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>