The previous approach ran efibootmgr inside the chroot, which only works
with arch-chroot (auto-mounts efivars) but fails silently with
systemd-nspawn or plain chroot. Move EFI boot entry creation to the host
where efivars is always available.
Also fixes wrong EFI loader path (\efi\EFI\... -> \EFI\...) and uses
the correct vendor label (e.g. "redhat" instead of raw os variable).
For non-RHEL distros, grub-install now uses --no-nvram to avoid
redundant NVRAM writes; the host efibootmgr handles entry creation
for all distros uniformly with idempotent pre-check.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
