[explorer] Temporary switch to be.cash chronik instance

Summary: The fabien.cash chronik is having some issues at the momemnt. Switching to the https://chronik.be.cash/xec until it is back online

Test Plan: Preview the site with @bot preview-explorer and check it is running correctly

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14089