Use new userlib.js instead of js-user-library
The attribute was moved to `userlib.js` (from `js-user-library`).
{ pkgs ? (import ../. {}).pkgs }:
-let js-user-library = pkgs.dfinity-sdk.packages.js-user-library; in
+let js-user-library = pkgs.dfinity-sdk.packages.userlib.js; in
pkgs.mkCiShell {
name = "dfinity-js-user-library-env";
