Feodra 34 and thus RHEL 9 switched to a unified grub configuration,
which means that the main grub config is always located in the same
location, /boot/grub2/grub.cfg.[1] osbuild has used this scheme for
hybrid boot on x64 but not on pure efi systems like aarch64. The
new osbuild option `uefi.unified` was introduced to select that new
unified grug cfg scheme also for those, pure efi, systems. Use that.
[1] https://fedoraproject.org/wiki/Changes/UnifyGrubConfig
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