capi packer qemu build no longer builds qemu images; unconditionally builds vagrant boxes

[carstenkoester]

Steps:

git clone https://github.com/kubernetes-sigs/image-builder
git checkout fda34643 # HEAD at the time this issue was reported
cd image-builder/images/capi
make build-qemu-ubuntu-2004

The build artifact used to be (and is expected to be) a qcow2 image in the output/ directory, but now always builds a Vagrant box:

...
==> ubuntu-2004: Gracefully halting virtual machine...
==> ubuntu-2004: Converting hard drive...
==> ubuntu-2004: Running post-processor: vagrant
==> ubuntu-2004 (vagrant): Creating a dummy Vagrant box to ensure the host system can create one correctly
==> ubuntu-2004 (vagrant): Creating Vagrant box for 'libvirt' provider
    ubuntu-2004 (vagrant): Copying from artifact: output/ubuntu-2004-kube-v1.17.11/ubuntu-2004-kube-v1.17.11
    ubuntu-2004 (vagrant): Compressing: Vagrantfile
    ubuntu-2004 (vagrant): Compressing: box.img
    ubuntu-2004 (vagrant): Compressing: metadata.json
Build 'ubuntu-2004' finished after 11 minutes 49 seconds.

# ls output/
# ls *box
packer_ubuntu-2004_libvirt.box

Checking out an earlier revision, df55d6f, restores the behavior of make build-qemu-ubuntu-2004 actually building qemu/qcow2 artifacts.

It seems like the post-processor was recently added to packer.json,

$ git blame -f -s packer.json
...
d8e1b6b23 images/capi/packer/qemu/packer.json  96)     }
892282320 images/capi/packer/qemu/packer.json  97)   ],
892282320 images/capi/packer/qemu/packer.json  98)   "post-processors": [
892282320 images/capi/packer/qemu/packer.json  99)     {
6ab0a4feb images/capi/packer/qemu/packer.json 100)       "type": "vagrant"
892282320 images/capi/packer/qemu/packer.json 101)     }
d8e1b6b23 images/capi/packer/qemu/packer.json 102)   ]
d8e1b6b23 images/capi/packer/qemu/packer.json 103) }

with 8922823 and 6ab0a4f both being merged yesterday (as part of #248).

Manual workaround is to either remove the vagrant post-processor, or add keep_input_artifact = true to it so that it preserves the original qcow2 image.