[ecash-herald] patch cron bug to send daily summaries at midnight UTC

Summary: Send every day not every hour

Test Plan: confirm in https://www.npmjs.com/package/cron docs

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16969