add way to get CI builds for a specific commit

I often find it useful to use to directly download the CI build outputs when reviewing a PR to base. My usual workflow is to click on `Details` on the particular tester for my platform, go to the `Build Properties` tab and copy the `download_url` there. I then download that tarball, extract it and run it. It would be really nice if I could just do `juliaup add ` and `juliaup` did everything else for me, even clean it up once I don't need it anymore. I think this should be possible since the corresponding download URLs (I think?) all follow the same scheme based on the commit hash.

Start by tracing the existing `juliaup add` command and its download handling. Determine how the CI `download_url` is derived from a commit SHA, then verify that the requested flow downloads, extracts, and later cleans up the selected build.

Written by the indexing model from the issue text.