run: copy artifacts with --sparse=never

On macOS, files copied on a bind mounted volume can be corrupted if the
original is sparse. In particular, this can happen for the rootfs image.
Setting --sparse=never in the copy prevents this corruption.

The rootfs.ext2 resulting from the current buildroot.yaml is not sparse,
so this change will only noticeably affect custom configurations.

Signed-off-by: Chris Reed <chris.reed@arm.com>