assert: replace var with let in lib/assert.js

Refs: https://github.com/nodejs/code-and-learn/issues/97

PR-URL: https://github.com/nodejs/node/pull/30261


Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>