Add ci_runner_machines foreign key to build tables

Create database migration to add ci_runner_machines.machine_xid foreign key to ci_builds tables as described in https://docs.gitlab.com/ee/architecture/blueprints/runner_tokens/#runner-identification-in-ci-jobs. I believe this is the only table that needs this FK in order to know exactly in which machine a job executed.

This page may contain information related to upcoming products, features and functionality. It is important to note that the information presented is for informational purposes only, so please do not rely on the information for purchasing or planning purposes. Just like with all projects, the items mentioned on the page are subject to change or delay, and the development, release, and timing of any products, features, or functionality remain at the sole discretion of GitLab Inc.