[ci] default.nix: do not override kapack's pkgs

ci/default.nix

 let
   pkgs = import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/18.03.tar.gz") {};
   kapack = import
-    ( fetchTarball "https://github.com/oar-team/kapack/archive/master.tar.gz")
-  { inherit pkgs; };
+    ( fetchTarball "https://github.com/oar-team/kapack/archive/master.tar.gz") {};
 in
 
 let