control packer error behaviour during fat image build

Author: sjpb

.github/workflows/fatimage.yml

@@ -47,7 +47,7 @@ jobs:
           . environments/.stackhpc/activate
           cd packer/
           packer init .
-          PACKER_LOG=1 packer build -only openstack.openhpc -on-error=ask -var-file=$PKR_VAR_environment_root/${{ vars.CI_CLOUD }}.pkrvars.hcl openstack.pkr.hcl
+          PACKER_LOG=1 packer build -only openstack.openhpc -on-error=${{ vars.PACKER_ON_ERROR }} -var-file=$PKR_VAR_environment_root/${{ vars.CI_CLOUD }}.pkrvars.hcl openstack.pkr.hcl
 
       - name: Get created image name from manifest
         id: manifest