proxmox-boot: fix #3632 copy kernel+initrd unconditionally

proxmox-boot: fix #3632 copy kernel+initrd unconditionally

do not use the -u (update) flag when copying kernel images and inird
from /boot to the ESPs:
* the ESPs are formatted with vfat, which has a 2 second precision for
  mtime (`linux/fs/fat/misc.c` - `fat_truncate_time`)
* cp -u compares the mtimes of source (kernel image in /boot not on
  vfat) and destination - leading to the copy always being carried
  out, if the source files remain the same (and do not happen to have
  a mtime exactly happening on a even second)

as laid out in the bug-report - the case where this leads to an
unbootable system is when a kernel-version is shipped twice (built
with different tool-chains) - e.g. currently the 5.11 kernels in PVE 6
and PVE 7.

tested the behavior of `cp -u` by running opensnopp-bpfcc and copying
a file twice onto ext4 (opened only once) and on vfat (opened twice).

additionally reproduced the issue (by dist-upgrading a PVE 6 VM to 7
with the pve-no-subscription repo) and verified this patch fixes it.

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