When I download it, how can I strip the _build/ component? This seems awkwardly not possible today, and risks moving other .pkg files that aren’t related to this job.
I was able to find some builds related to this in your org. The _build/ is the file path to the artifact from the upload step. So if you want to do an artifact download you would need to reference the same path. And because you used *.pkg. it will copy the files in path with the extension. You can give the artifacts a specific name from the upload step so it would be easier to differentiate.