vendor/cmd/go: skip TestDefaultGOPATH

It seems to have been broken by CL 104975: we were invoking `vgo env`
with a GOROOT that did not have a complete source tree.

Updates golang/go#25501.

Change-Id: I238f710bc094cbae9381e3fdd49d5c1f999b5ea5
Reviewed-on: https://go-review.googlesource.com/117035
Reviewed-by: Russ Cox <rsc@golang.org>
diff --git a/vendor/cmd/go/go_test.go b/vendor/cmd/go/go_test.go
index d0d7ed0..f354395 100644
--- a/vendor/cmd/go/go_test.go
+++ b/vendor/cmd/go/go_test.go
@@ -1963,6 +1963,8 @@
 }
 
 func TestDefaultGOPATH(t *testing.T) {
+	t.Skip("vgo") // Needs a more realistic GOROOT; see RuntimeGoroot below.
+
 	tg := testgo(t)
 	defer tg.cleanup()
 	tg.parallel()