How can I share coverage information between build stages in PRs from forks?

I’m currently experimenting with this hack: https://twitter.com/webKnjaZ/status/1097078272867921920

It’s just frustrating that there’s not build-wide shared space, then I wouldn’t need any third party service or any hackery. The cache is potentially huge, and I’m talking about a few kb of coverage information here…

Feature request in: Shared disk space between all jobs in a build