Restore Dockerfile ENV var SAST_ANALYZER_IMAGE_TAG

What does this MR do?

Restores Dockerfile ENV var SAST_ANALYZER_IMAGE_TAG. Should be applied after the workaround is merged that prevents Dockerfile ENV vars being reset to an empty string.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/security-products/release/issues/75

Does this MR meet the acceptance criteria?

• [-] Changelog entry added
• [-] Documentation created/updated for GitLab EE, if necessary
• [-] Documentation created/updated for this project, if necessary
• [-] Documentation reviewed by technical writer or follow-up review issue created
• [-] Tests added for this feature/bug
• [-] Job definition updated, if necessary
  • [-] Job definition example
  • [-] CI Templates (to be removed)
  • [-] Vendored CI Templates (also in CE)
  • Ensure that all of the ENV vars supported are propagated to Docker containers upon docker run in job definitions
• Conforms to the code review guidelines
• [-] Conforms to the Go guidelines
• Security reports checked/validated by reviewer