Provide predictable latest ISO, bootstrap tarball and signature symlinks

Current structure on the mirrors has the following symlinks:

• archlinux-x86_64.iso→archlinux-${VERSION}-x86_64.iso,
• archlinux-x86_64.iso.sig→archlinux-${VERSION}-x86_64.iso.sig.
• archlinux-bootstrap-x86_64.tar.gz→archlinux-bootstrap-${VERSION}-x86_64.tar.gz,
• archlinux-bootstrap-x86_64.tar.gz.sig→archlinux-bootstrap-${VERSION}-x86_64.tar.gz.sig.

See https://github.com/pierres/archiso-manager/pull/10, https://github.com/pierres/archiso-manager/pull/11 and https://github.com/pierres/archiso-manager/pull/12 for details on why they exist.

arch-release-promotion should create these symlinks to not break existing expectations.

mentioned in merge request ci-scripts!6 (merged)

added feature parity with archiso-manager label

changed title from Provide predictable latest ISO and signature symlinks to Provide predictable latest ISO, bootstrap tarball and signature symlinks

changed the description

Arch infrastructure (infrastructure@4de01431) now depends on the archlinux-bootstrap-x86_64.tar.gz symlink.