src: replace `auto`s in node_contextify.cc

PR-URL: https://github.com/nodejs/node/pull/38644


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>