bootstrap/cbuild: fix yum repos for CentOS 8 Linux containers
CentOS 8 Linux is end of life.
The containers are now at https://quay.io/repository/centos/centos?tab=tags,
but their yum repos point to non-existing URLs and dnf upgrade
fails.
Patch the system before package update.
See a build failure here: https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/jobs/18776121
Edited by Thomas Haller