CentOS Templates are broken

CentOS 7 and 8 are now EOL for a while.

It's possible to workaround this by enabling the Vault repositories, but since the templates run yum update internally, this fails before FDO_DISTRIBUTION_EXEC runs.

+ for cmd in "${pkgcmd_upgrade[@]}"
+ buildah run --isolation chroot centos-working-container sh -c 'yum upgrade -y '

Like #45 (closed), FDO_DISTRIBUTION_PREXEC would fix this.