hydra: set system-features for usage as a remote builder

hosts/hydra/default.nix

@@ -46,6 +46,10 @@ in
       keep-outputs = true;
       max-jobs = 8;
       trusted-users = [ "hydra" "root" "@wheel" ];
+      system-features = [
+        "kvm" "big-parallel"
+        "nixos-test" "benchmark"
+      ];
     };
   };