Missing data dependency between prep and compile stages

[tmcgilchrist]

There is a missing data dependency between prep and compile stages when building package documentation. For example on cohttp-lwt-unix.4.1.1

When it tries to rsync the universes used in the compile step, sometimes they don't exist. The pipeline should know whether they exist and re-run the equivalent prep step to produce them. Manually re-running the right prep step unblocks the pipeline to continue.