Workhorse: switching to the sources bundled with the Rails app
This is cherry-picking in advance the changes that upstream did for 13.9.0rc42: we are validating heptapod#402 (closed) with GitLab 13.7 and only afterwards will merge 13.9
We're using a dedicated cache key in order not to pollute the
regular Heptapod 0.19 builds that will from now on be done from
the heptapod-stable
branch.