Make cargo run work again

I thought I'd tested this, but obviously I didn't. This will let Cargo
know, that `cargo run` should run `fj-app`.

Please note that the app itself is still broken, as it doesn't
understand where the `target/` directory is in the workspace.

Cargo.toml

@@ -11,3 +11,4 @@ members = [
 
     "release-operator",
 ]
+default-members = ["fj-app"]