cmd/go: skip cgo test when cgo isn't available

Fixes #26758

Change-Id: If08bafc85c353dfbb546107b75c8c884ab9c88e4
Reviewed-on: https://go-review.googlesource.com/127475
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/testdata/script/mod_case_cgo.txt

+[!cgo] skip
+
 env GO111MODULE=on
 
 go get rsc.io/CGO