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.

nixos/modules/config/system-path.nix

@@ -45,6 +45,7 @@ let
       pkgs.strace
       pkgs.su
       pkgs.time
+      pkgs.texinfoInteractive
       pkgs.utillinux
       extraManpages
     ];
@@ -138,6 +139,13 @@ in
           if [ -x $out/bin/update-desktop-database -a -w $out/share/applications ]; then
               $out/bin/update-desktop-database $out/share/applications
           fi
+
+          if [ -x $out/bin/install-info -a -w $out/share/info ]; then
+            shopt -s nullglob
+            for i in $out/share/info/*.info $out/share/info/*.info.gz; do
+                $out/bin/install-info $i $out/share/info/dir
+            done
+          fi
         '';
     };