go/build: fix test if built with CGO_ENABLED=0

Fixes #6567.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/14502060

src/pkg/go/build/deps_test.go

@@ -392,6 +392,9 @@ func TestDependencies(t *testing.T) {
 				if allowedErrors[osPkg{ctxt.GOOS, pkg}] {
 					continue
 				}
+				if !ctxt.CgoEnabled && pkg == "runtime/cgo" {
+					continue
+				}
 				// Some of the combinations we try might not
 				// be reasonable (like arm,plan9,cgo), so ignore
 				// errors for the auto-generated combinations.