Merge pull request #8217 from ip1981/mwlib

mwlib uses pdftk to create books

nixos/modules/services/misc/mwlib.nix

···
         chmod -Rc u=rwX,go= '${cfg.nslave.cachedir}'
       '';
 
-      path = with pkgs; [ imagemagick ];
+      path = with pkgs; [ imagemagick pdftk ];
       environment = {
         PYTHONPATH = concatMapStringsSep ":"
           (m: "${pypkgs.${m}}/lib/${python.libPrefix}/site-packages")