Fixing image variable for SAST analyzers

What does this MR do?

Fixes flawfinder, brakeman, and bandit SAST jobs not running. If the template goes live it would all pipelines using any of the three analyzers listed. The image variable had a missing variable expansion $ for the flawfinder, brakeman, and bandit analyzers that was introduced in the previous MR to the SAST template. This MR fixes that.

Failing job: https://gitlab.com/gitlab-org/security-products/tests/python-pipenv/-/jobs/706978119

Relates to #238080 (closed)

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Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done