There is an overlay for the `hydra` name, but `hydra_unstable` was used, which can refer to the nixpkgs package and lead to and outdated hydra version and requires configuring the correct package attribute downstream.
Nixpkgs only contains a `hydra_unstable`, not `hydra`, package, so
adjust the default accordingly, and then override it to our package in
the separate module which does that.
