ci: split & reuse debian version identifier
It was set on non-debian containers, and it was missing on debian-based rootfs containers.
Fixes: 40233010 ("ci: enable ci-deb-repo for libdrm 2.4.119 (and others in the future)")
It was set on non-debian containers, and it was missing on debian-based rootfs containers.
Fixes: 40233010 ("ci: enable ci-deb-repo for libdrm 2.4.119 (and others in the future)")