Update galaxy-importer ansible-test image for updated ansible-core

Update galaxy-importer ansible-test image https://github.com/ansible/galaxy-importer/blob/master/galaxy_importer/ansible_test/container/Dockerfile to ansible-core==2.12.4 to match that used in galaxy_ng repo. (Note: ansible-core version in galaxy_ng is independent from the galaxy-importer ansible-test image so they can be upgraded independently)