Update lib/attrsets.nix

     the *leaf* attributes of a potentially-nested attribute set:

     the second argument of the function will never be an attrset.

     Also, the first argument of the argument function is a *list*

     of the names of the containing attributes.

     of the attribute names that form the path to the leaf attribute.

     For a function that gives you control over what counts as a leaf,

     see `mapAttrsRecursiveCond`.

     Example:

       mapAttrsRecursive (path: value: concatStringsSep "-" (path ++ [value]))