I want to focus on configuration management first. We can use NixOS for the Linux runners, and there's a powershell script for the Windows runners, but afaik there's nothing for FreeBSD.
Big picture:
Currently we're at step 1, prototyping the configuration of a FreeBSD system.
In order to speed up the feedback cycle, I have ripped everything out of GHC that isn't related to running the FreeBSD testsuite. These changes will not be merged. While prototyping, the steps described in the discussion are where the important information is. Not the code.