cmd/stunstamp: increase probe jitter (#13362)

We've added more probe targets recently which has resulted in more
timeouts behind restrictive NATs in localized testing that don't
like how many flows we are creating at once. Not so much an issue
for datacenter or cloud-hosted deployments.

Updates tailscale/corp#22114

Signed-off-by: Jordan Whited <jordan@tailscale.com>

cmd/stunstamp/stunstamp.go

@@ -380,7 +380,7 @@ func probe(meta nodeMeta, cf *connAndMeasureFn, dstPort int) (*time.Duration, er
 		Port: dstPort,
 	}
 
-	time.Sleep(rand.N(200 * time.Millisecond)) // jitter across tx
+	time.Sleep(rand.N(400 * time.Millisecond)) // jitter across tx
 	rtt, err := cf.fn(cf.conn, meta.hostname, netip.AddrPortFrom(meta.addr, uint16(dstPort)))
 	if err != nil {
 		if isTemporaryOrTimeoutErr(err) {