.gitlab-ci.yml: Add 'after_script' stage to prep for artifact collection

.gitlab-ci.yml: Add 'after_script' stage to prep for artifact collection

This is one of the preparation steps that if not done would otherwise
collide with local container executions where we:
1) don't collect artifacts
2) are not limited by GitLab's environment and hence moving build
   artifacts to unusual places would only cause confusion when doing
   local build inspection in an interactive container shell session

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>