Use explicit image: in template

What does this MR do?

Adds explicit image: in lib/gitlab/ci/templates/Security/License-Management.gitlab-ci.yml

See also 36f80ead (comment 338611182)

Screenshots

https://staging.gitlab.com/tkuah/image-test/-/jobs/37201447

No changelog necessary as builds on top of !30645 (merged) in same milestone