[ecash-lib] Install deps from npm before publishing

Summary:
This follows the same approach used for chronik-client. Installers of the published npm module ecash-lib cannot get its required dependencies from the monorepo in the same way that abc monorepo apps can.

So, before we publish, we replace the local deps with the latest published deps.

Test Plan: docker build -f ecash-lib.Dockerfile -t ecash-lib_local . builds

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17215