Update packer scripts to remove disks on termination

[ScottGuymer]

Responding to myself after noticing by looking at the AMI that it had "Delete on termination" set to "No"

In /mages/ubuntu-focal/github_agent.ubuntu.pkr.hcl, I updated to:

  launch_block_device_mappings {
    device_name           = "/dev/sda1"
    volume_size           = var.root_volume_size_gb
    volume_type           = "gp3"
    delete_on_termination = true
  }

Originally posted by @Fgerthoffert in https://github.com/philips-labs/terraform-aws-github-runner/discussions/1742#discussioncomment-2158915

[toast-gear]

Yeh, we noticed this today and were going to raise a PR to fix it tomorrow (I don't mind who raises the PR). A default of true makes much more sense than false for GitHub Actions runners, apparently the default in Packer is false.