[explorer] Temporary update to chronik instance

Summary: changing the chronik instance to https://chronik.be.cash/xec as it will resolve first in this downtime

Test Plan: run and see if it doesnt work

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D13140