odhcp-script: delete stale dirs from previous runs

This requires adding LFS as a dependency because native Lua has
no way to iterate a directory, but it seems to be Not Huge and
hopefully we'll have other uses for it

pkgs/fennelrepl/default.nix

@@ -14,6 +14,7 @@ let packages = [
       linotify
       anoia
       fennel
+      lua.pkgs.luafilesystem
     ];
     join = ps: builtins.concatStringsSep ";" ps;
     luapath = join (builtins.map (f: