feat: hint tsnet to make a login

If I don't set this var, I get the message "Authkey is set; but state
is NoState. Ignoring authkey. Re-run with TSNET_FORCE_LOGIN=1 to
force use of authkey."
The thing to note here is the program runs fine even in the
absence of this var.

main.go

@@ -9,6 +9,7 @@ import (
 	"io"
 	"log"
 	"net/http"
+	"os"
 
 	"github.com/r3labs/sse/v2"
 	"tailscale.com/tsnet"
@@ -107,6 +108,8 @@ func (h *assetsHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 }
 
 func main() {
+	os.Setenv("TSNET_FORCE_LOGIN", "1")
+
 	s := &tsnet.Server{
 		Hostname: "telltail",
 	}