flake: set a default package

This allows `nix build` to run, without needing to use `nix build
.#tailscale`.

Signed-off-by: Cole Helbling <cole.helbling@determinate.systems>

flake.nix

@@ -89,6 +89,7 @@
       ts = tailscale pkgs;
     in {
       packages = {
+        default = ts;
         tailscale = ts;
       };
       devShell = pkgs.mkShell {