Debian 12 ISO URL no longer valid

With the release of Debian 13 (“Trixie”), Debian 12 images have been moved from the current directory to the Debian archive.

As a result, the ISO URL used in debian-12.pkr.hcl is now broken:

variable "iso_url" {
  type    = string
  default = "https://cdimage.debian.org/debian-cd/current/amd64/iso-cd/debian-12.8.0-amd64-netinst.iso"
}

Affected file: https://gitlab.cs.pub.ro/SCGC/packer/-/blob/master/debian-12.pkr.hcl?ref_type=heads#L18

Proposed fix: Update the default value of iso_url to reference the Debian 12 archive path so builds work again - https://cdimage.debian.org/cdimage/archive/12.8.0/amd64/iso-cd/