Getting this to work would be so awesome!
Unfortunately I got stuck at some linker issues when trying to use Clang_jll, which was a bit above my abilities. Getting make to work was no issue though, and I think there are already standalone stanc3 binaries available on the stanc3 repository.
Thanks. I actually figured out how to make a cached artifact on Gitlab for it, but cmdstan compiled is actually so huge (around 1.1 GB I last checked) that downloading the cached artifact is slower in CI than building it via Github Actions (15 min vs 2-4 min).
Now I am not so sure that pursuing generating an artifact makes sense. Not for something this large.
Just to provide an update to this, recent versions of Stan support installation via conda, so it is pretty easy to bundle a CmdStan installation with a project using CondaPkg.jl.