dasel: set mainProgram

pkgs/applications/misc/dasel/default.nix

···
     homepage = "https://github.com/TomWright/dasel";
     changelog = "https://github.com/TomWright/dasel/blob/v${version}/CHANGELOG.md";
     license = licenses.mit;
+    mainProgram = "dasel";
     maintainers = with maintainers; [ _0x4A6F ];
   };
 }