Following up with that, it seems that the failure is due to a change in
the kernel.
I'm testing 2 Kali netinst ISOs, one from last week (no problem), and a
daily iso from today (which fails).
Last weekly iso had kernel 6.1.0-kali9-amd64. In this iso, `cat
/proc/filesystems | grep efi` gives nothing, hence the grub-installer
postinst doesn't try to mount the efivarfs.
Today's iso has kernel 6.3.0-kali1-amd64. In this iso, `cat
/proc/filesystems | grep efi` gives `nodev efivarfs`, hence the
grub-installer postinst tries to mount efivarfs, and fails.
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Arnaud Rebillout / OffSec / Kali Linux Developer