ci: re-enable cache and use plugin fork
Created by: jhchabran
Follow-up to INC-101. See also https://github.com/sourcegraph/infrastructure/pull/3316 and https://github.com/jhchabran/cache-buildkite-plugin/commit/6652014df9051ad71e988d6bfd0a39ced47dd567
Test plan
Main-dry-run + yarn cache working