awscli: Set meta.mainProgram

… so that `nix run nixpkgs#awscli` works

pkgs/tools/admin/awscli/default.nix

···
     homepage = "https://aws.amazon.com/cli/";
     description = "Unified tool to manage your AWS services";
     license = licenses.asl20;
+    mainProgram = "aws";
     maintainers = with maintainers; [ ];
   };
 }