fix: installer fails when systemd-boot is used instead of GRUB2

[alehostert]

Error

{"level":"error","time":"2026-07-01T04:58:23Z","message":"DisableSelinuxFailed: {\"stdErr\":\"sed: can't read /etc/default/grub: No such file or directory\\n\",\"exitCode\":2}"}

Scenario

The error occurs during Ez installation on OpenSUSE MicroOS. When choosing the bootloader, systemd-boot was selected instead of GRUB2 for EFI. The installer then stopped and displayed the message above.

Workaround

Use grub2 before running the installer.

Proposals

1. Adapt the Ez installer to work with both GRUB and systemd-boot; or
2. Document that GRUB is a mandatory requirement.