Accept the nonfree licensed package we require to monitor our RAID arrays

Thanks @tomprince!

morph/lib/default.nix

@@ -26,7 +26,11 @@
     # Ensure that configuration of the system where this runs
     # doesn't leak into what we build.
     # See https://github.com/NixOS/nixpkgs/issues/62513
-    config = {};
+    config = { pkgs }: let lib = pkgs.lib; in {
+      allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
+        "megacli"
+      ];
+    };
     overlays = [];
   };
 }