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-tests.nix

@@ -7,7 +7,7 @@ let
     {
       imports = [
         nixosModules.hydra
-        nixosModules.overlayNixpkgsForThisHyydra
+        nixosModules.overlayNixpkgsForThisHydra
         nixosModules.hydraTest
       ];