Fix sparse vmlinuz boot failure on XFS by changing cp --sparse to auto

[liulanze]

Fix boot failure caused by sparse vmlinuz kernel files on XFS partitions.

When Image Customizer repartitions an image (via storage: config), it copies
all files from the old image to the new one using cp --sparse always. This
flag aggressively scans file content for zero-byte runs and converts them into
filesystem holes.

The AZL 3.0 vmlinuz kernel binary contains a legitimate run of zero bytes.
cp --sparse always turns this into a sparse hole in the destination file.
GRUB cannot read sparse files on XFS, causing a boot failure.

---

Verification

Reproduced and verified with Azure Linux 3.0 core-efi image
(core-3.0.20260401.vhdx) using the partitions-xfs-boot.yaml test config.

Before fix (--sparse always):

filefrag -v /mnt/boot/vmlinuz-6.6.130.1-3.azl3
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..    3566:      34935..     38501:   3567:
   1:     3568..    3702:      38503..     38637:    135:             last,eof

Logical block 3567 is a sparse hole.

After fix (--sparse auto):

filefrag -v /mnt/boot/vmlinuz-6.6.130.1-3.azl3
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..    3702:      31809..     35511:   3703:             last,eof

Single contiguous extent, no holes.