tsnet/examples/tshello: update example for LocalClient method (#5966)

Before this would silently fail if this program was running on a machine
that was not already running Tailscale. This patch changes the WhoIs
call to use the tsnet.Server LocalClient instead of the global tailscale
LocalClient.

Signed-off-by: Xe <xe@tailscale.com>

Change-Id: Ieb830fbce81292acc4c3b4d1b675aa10766a18dc
Signed-off-by: Xe <xe@tailscale.com>
(cherry picked from commit 86c5bddce2)

tsnet/example/tshello/tshello.go

@@ -29,13 +29,20 @@ func main() {
 	if err != nil {
 		log.Fatal(err)
 	}
+	defer ln.Close()
+
+	lc, err := s.LocalClient()
+	if err != nil {
+		log.Fatal(err)
+	}
+
 	if *addr == ":443" {
 		ln = tls.NewListener(ln, &tls.Config{
 			GetCertificate: tailscale.GetCertificate,
 		})
 	}
 	log.Fatal(http.Serve(ln, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		who, err := tailscale.WhoIs(r.Context(), r.RemoteAddr)
+		who, err := lc.WhoIs(r.Context(), r.RemoteAddr)
 		if err != nil {
 			http.Error(w, err.Error(), 500)
 			return