ci: potfile: Add 'variables' to definition

ci: potfile: Add 'variables' to definition

The 'potfile' job is lacking the 'variables' section which defines the
'NAME' variable used by the 'image' property resulting in the 'potfile'
job failing with:

 Pulling docker image registry.gitlab.com/libvirt/libvirt/ci-:latest ...
 WARNING: Failed to pull image with policy "always": invalid reference format (manager.go:235:0s)
 ERROR: Job failed: failed to pull image "registry.gitlab.com/libvirt/libvirt/ci-:latest" with specified policies [always]: invalid reference format (manager.go:235:0s)

Fixes: 491d918502e50bf15c75d183bb41e3c0de5a0e1b
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>