cmd/go: skip broken TestExecutableGOROOT/RelocatedExe

Test just doesn't work.

Updates #20284

Change-Id: I31c3f771e68dcdc471767594a1467e19fbf5fe88
Reviewed-on: https://go-review.googlesource.com/46830
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@google.com>

src/cmd/go/go_test.go

@@ -4072,6 +4072,8 @@ func TestExecutableGOROOT(t *testing.T) {
 	newRoot := tg.path("new")
 
 	t.Run("RelocatedExe", func(t *testing.T) {
+		t.Skip("TODO: skipping known broken test; see golang.org/issue/20284")
+
 		// Should fall back to default location in binary.
 		// No way to dig out other than look at source code.
 		data, err := ioutil.ReadFile("../../runtime/internal/sys/zversion.go")