Align the partition end sectors and add partition names

sgdisk by default only aligns the partition start. Use the --align-end option to align the partition end too. See https://wiki.archlinux.org/title/Advanced_Format#Partition_alignment. Just to note, this doesn't make the images usable on 4Kn disks.

Additionally:

• Add PARTLABEL values to all partitions.
• Do not hardcode the partition number and end sectors in sgdisk commands by using the special value "0".
• Follow the Discoverable Partitions Specification by using the "Linux x86-64 root" partition type.