cmd/go: disable TestIssue10952 without external network

Should fix arm64 build failure.

Change-Id: Ib35e4a69a1082e461a1eddf3265544a1d2ff98e5
Reviewed-on: https://go-review.googlesource.com/11710Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/go/go_test.go

@@ -984,6 +984,8 @@ func TestImportCommentConflict(t *testing.T) {
 
 // cmd/go: custom import path checking should not apply to github.com/xxx/yyy.
 func TestIssue10952(t *testing.T) {
+	testenv.MustHaveExternalNetwork(t)
+
 	if _, err := exec.LookPath("git"); err != nil {
 		t.Skip("skipping because git binary not found")
 	}