Try again to ensure hydra module is usable

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.

nixos-modules/default.nix

@@ -3,8 +3,9 @@
 {
   hydra = import ./hydra.nix;
 
-  overlayNixpkgsForThisHyydra = {
+  overlayNixpkgsForThisHydra = { pkgs, ... }: {
     nixpkgs = { inherit overlays; };
+    services.hydra.package = pkgs.hydra;
   };
 
   hydraTest = { pkgs, ... }: {