modify grub during install

[SeanBE]

https://unix.stackexchange.com/questions/533244/hide-grub-menu-at-boot-on-fedora-30

https://jamielinux.com/blog/hide-grub-menu-on-fedora/

[SeanBE]

and add systemd.unified_cgroup_hierarchy=0 to GRUB_CMDLINE_LINUX in /etc/default/grub + run grub2-mkconfig with a reboot.