[keyserver] use yarn instead of npm for running prod

Summary:
Was originally encountering errors when running my keyserver image in an ecs task. Switching from npm to yarn fixed the issue

Depends on D12560

Test Plan: Successfully able to run local keyserver and ecs tasks successfully run keyserver image

Reviewers: bartek, ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12687