runtime: skip TestPingPongHog on builders

This test is failing consistently in the longtest builders,
potentially masking regressions in other packages.

Updates #35271

Change-Id: Idc03171c0109b5c8d4913e0af2078c1115666897
Reviewed-on: https://go-review.googlesource.com/c/go/+/206098Reviewed-by: Carlos Amedee <carlos@golang.org>

src/runtime/proc_test.go

@@ -6,6 +6,7 @@ package runtime_test
 
 import (
 	"fmt"
+	"internal/testenv"
 	"math"
 	"net"
 	"runtime"
@@ -422,6 +423,7 @@ func TestPingPongHog(t *testing.T) {
 	if testing.Short() {
 		t.Skip("skipping in -short mode")
 	}
+	testenv.SkipFlaky(t, 35271)
 
 	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(1))
 	done := make(chan bool)