Bug#1031183: grub-installer: postinst fails if efivarfs cannot be mounted

[Arnaud Rebillout <arnaudr@kali.org>]

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