cmd/go: disable gofuzzbeta build tag by default

cmd/go: disable gofuzzbeta build tag by default

It was enabled by default on the dev.fuzz branch so that users could
check in fuzz targets without breaking their builds.

Now that dev.fuzz is merged to master, users should switch to a go1.18
release constraint instead.

For #47037

Change-Id: I0475368eb4570fe0c7027a3ae8f6950449c1a9ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/351118
Trust: Jay Conrod <jayconrod@google.com>
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>