Use new SAST variable for SAST images

What does this MR do?

This MR replaces the current image.name in the SAST vendored template with a image.name that is override-able via setting a SAST_ANALYZER_IMAGE variable.

Related issue #238080 (closed)

Demo:

• Parent pipeline
  • tmp image tagged and pushed: Successfully tagged registry.gitlab.com/gitlab-org/security-products/analyzers/gosec/tmp:dc4ab6d2fa5970f081de0610309b5a32aeeec988
• Job pulling from correct tmp image
  • Pulling docker image registry.gitlab.com/gitlab-org/security-products/analyzers/gosec/tmp:dc4ab6d2fa5970f081de0610309b5a32aeeec988 ...

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