convert Ubuntu AMI from MBR partitioning to GPT partitioning

packer.json

// the following assumes AWS nitro (e.g. t3.*, m5.*, &c) instances (which use /dev/nvme0n1 as root disk)
// "safely" switches from MBR to GPT partitioning in Ubuntu < 21.04 or other AMIS. 
// Tested using gdisk 1.0.3 from 18.04 / Bionic.
// after the change, you can use >2TB root disks. either initially, or by resizing a smaller one -- in nitro and non-nitro types

provisioners: [
   {
      "type": "shell",
      "inline": [
        "echo Converting to GPT - AWS nitro type instance",
        "sudo sgdisk /dev/nvme0n1 --mbrtogpt --new=2:34:2047 --typecode=2:ef02",
        "sudo partprobe",
        "sudo grub-install /dev/nvme0n1"
      ]
    }
]