nixos: generate infodirs directly in system-path

`man 1 info` says:

   The first non-option argument, if present, is the menu entry to
   start from; it is searched for in all `dir' files along INFOPATH.
   If it is not present, info merges all `dir' files and shows the
   result. Any remaining arguments are treated as the names of menu
   items relative to the initial node visited.

Which means that this does what previous programs/info did and #8519
(on-the-fly infodir generation for Emacs) wanted to do, but for both
programs.