nixos/system-path.nix: add 3 to every priority

We can’t use lowPrio here because it erases the differences in
priority of the packages by setting it to a constant value. see this
comment for info:

https://github.com/NixOS/nixpkgs/issues/55886#issuecomment-464766877

nixos/modules/config/system-path.nix

@@ -7,7 +7,7 @@ with lib;
 
 let
 
-  requiredPackages = map lib.lowPrio
+  requiredPackages = map (pkg: setPrio ((pkg.meta.priority or 5) + 3) pkg)
     [ config.nix.package
       pkgs.acl
       pkgs.attr