Ok this all works now, but the GHA stuff isn't great.
[ ] Workout how to reference enviroment variable in path (should be env.REFERENCE_TESTS_...
[ ] Workout how to give a different folder per matrix combine (should be interpolating things into the enviroment variable)
[ ] Workout how to give a path that works on linux and windows (I think i can't use ~, but when I don't it doesn't find it, probably because it is in a subdirectory. Probably an absolute path is best)
[ ] Document all this, including where to find the artifact. and the fact that it is 1 artifact per workflow, and that it doesn't show up til all are done
Closes https://github.com/JuliaTesting/ReferenceTests.jl/issues/79