runtime/debug: mark TestSetGCPercent as flaky

Updates #20076

Change-Id: I4eb98abbb49174cc6433e5da2c3660893ef88fd1
Reviewed-on: https://go-review.googlesource.com/41615
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/debug/garbage_test.go

@@ -5,6 +5,7 @@
 package debug_test
 
 import (
+	"internal/testenv"
 	"runtime"
 	. "runtime/debug"
 	"testing"
@@ -110,6 +111,8 @@ var (
 )
 
 func TestSetGCPercent(t *testing.T) {
+	testenv.SkipFlaky(t, 20076)
+
 	// Test that the variable is being set and returned correctly.
 	old := SetGCPercent(123)
 	new := SetGCPercent(old)