Make mirror and remote directory for bootstrap images (more) configurable

[dvzrv]

We are currently discussing the cleanup of our release artifact directories (https://gitlab.archlinux.org/archlinux/releng/-/issues/11). One project affected by this change would be archlinux-repro, as the mirror and remote directory for the bootstrap image are currently only overridable via BOOTSTRAPMIRROR. While technically this suffices, the directory structure will soon be more explicit (in regards to per-type artifact directory structure).

To make this a little more flexible I propose to decouple the mirror and the directory from one another. A similar approach has been chosen for releng (which relies on the iso release artifact).

This ticket is here to track the progress for this change for archlinux-repro and to propose an implementation for this situation.