[chronik-client] Specify a peer dep that pnpm struggles to resolve in monorepo environments

Summary:
Have been looking at pnpm as a solution for the mess that is js dependencies. One significant plus is that it completely removes all individual package-lock.json files, replacing instead with an easier-to-read top-level yaml file.

The migration is ... challenging, esp with CI considerations. But the benefit is real. No more package-lock.json files in diff review...pretty nice.

Anyway one haunted issue I came across was apps that use chronik-client being built in CI and not being able to find long. Specifying it here "should" help that.

Test Plan: land, should cause no harm as we already have this dep. Then I can test CI builds. I could test by publishing an rc, but bc this change is kind of a no-op, figured ok to just give it a go.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18831