proxmox-boot: fix #3729 add --graceful to bootctl invocation

proxmox-boot: fix #3729 add --graceful to bootctl invocation

The version of systemd boot in bullseye, tries writing an efivar which
is not writeable on certain (broken) UEFIs (HP thin clients).

The issue was not present in the version in buster (the variable
simply did not get written) and can be worked around by adding
--graceful to the `bootctl install` command.

see also:
https://github.com/systemd/systemd/issues/13603

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>