I don't have much experience with GitHub's CI offering. But if this is an accurate description of the steps you need to take to use it securely ... then I don't think it can, in fact, ever be used securely.

Even if you trust Microsoft's cloud engineering on the backend, this is a system that does not appear to follow even the most basic principles of privilege and isolation? I'm not sure why you would even try to build "supply-chain security" on top of this.