EFI support: fix for grub-install usage with efivarfs

We had to apply fixes due to efivars vs efivarfs in Debian kernel
versions >=5.10, and addressed them in commit d91d9f3e.

Those changes were incomplete though: while efibootmgr was invoked in
the according and expected environment, invocation of grub-install
doesn't reliably work (as at that time /sys/firmware/efi/efivars is no
longer accessible).

GRUB installation on EFI systems without /sys/firmware/efi/efivars
present warns with (also see https://sources.debian.org/src/grub2/2.04-20/debian/patches/efi-variable-storage-minimise-writes.patch/?hl=650#L650):

| EFI variables are not supported on this system

But it returns with exit code 0. This leaves us with an incomplete and
therefore not booting GRUB EFI environment. Fix it by unmounting
/sys/firmware/efi/efivars only from within our final stage.