[ecash-herald] Make sure to build mock-chronik-client

Summary: mock-chronik-client is typescript now and has a build step

Test Plan: docker build -f ecash-herald.Dockerfile -t ecash-herald_local .

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17362