cmd/tailscale/cli: add netcheck dev knob TS_DEBUG_NETCHECK_UDP_BIND

cmd/tailscale/cli/netcheck.go

@@ -43,7 +43,9 @@ var netcheckArgs struct {
 }
 
 func runNetcheck(ctx context.Context, args []string) error {
-	c := &netcheck.Client{}
+	c := &netcheck.Client{
+		UDPBindAddr: os.Getenv("TS_DEBUG_NETCHECK_UDP_BIND"),
+	}
 	if netcheckArgs.verbose {
 		c.Logf = logger.WithPrefix(log.Printf, "netcheck: ")
 		c.Verbose = true