Cirrus: Disable image build job abort on push

Normally cirrus will abort jobs if another push is made to a branch.
However, with image builds, other VMs are created/managed by packer.
Therefor if cirrus aborts a task, it's possible some packer managed VMs
will be left behind.  Disable this behavior for image-building only.

Signed-off-by: Chris Evich <cevich@redhat.com>

.cirrus.yml

@@ -99,6 +99,8 @@ build_vm_images_task:
         # Version of packer to use
         PACKER_VER: "1.3.1"
 
+    # VMs created by packer are not cleaned up by cirrus
+    auto_cancellation: $CI != "true"
 
     gce_instance:
         image_name: "image-builder-image"  # Simply CentOS 7 + packer dependencies