nix: upgrade readTree

Change-Id: I460800dc3d8095e2ae89b8bd6ed7c5f0c90b6ccf
diff --git a/shell.nix b/shell.nix
index de3db3c..04b8501 100644
--- a/shell.nix
+++ b/shell.nix
@@ -4,7 +4,7 @@
 
   hscloud = import ./default.nix {};
 
-in with hscloud.config.pkgs; let
+in with hscloud.pkgs; let
 
   wrapper = pkgs.writeScript "wrapper.sh"
   ''