Lock down CLAIR_DB_IMAGE_TAG for consistent tests

Each time a test is run, the latest version of the arminc/clair-db vulnerabilities db is retrieved by the Container-Scanning.gitlab-ci.yml template, which means that the vulnerability report created during the test stage will constantly change, leading to inconsistent test results.

This MR fixes this issue by hard coding the CLAIR_DB_IMAGE_TAG variable the same way the container scanning test project does it.

This MR relies on gitlab-org/security-products/ci-templates!48 (merged) being merged first