cmd/go: skip gotoolchain_net script test for new-ish go experiments

cmd/go: skip gotoolchain_net script test for new-ish go experiments

Skip the gotoolchain_net script test if any new-ish GOEXPERIMENT is
set, since it will fail under these circumstances.

Updates #62106.

Change-Id: Idfef8af7246a3fbe6481347872a94fe64bbaed7f
Reviewed-on: https://go-review.googlesource.com/c/go/+/519195
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>