Leapp fails on s390x if a /etc/grub.d directory is present

Issue

The below error is reported by Leapp:

2023-01-03 11:28:56.408140 [ERROR] Actor: source_boot_loader_scanner
Message: Failed to call `grubby` to list available boot entries.
Summary:
    Details: Command ['grubby', '--info', 'ALL'] failed with exit code 1.
    Stderr: error opening /boot/grub/grub.cfg for read: No such file or directory