ocamlPackages.alcotest-mirage: use Dune 3

pkgs/development/ocaml-modules/alcotest/mirage.nix

···
 
   inherit (alcotest) version src;
 
+  duneVersion = "3";
+
   propagatedBuildInputs = [ alcotest lwt logs mirage-clock duration ];
 
   doCheck = true;