Overriding SAST_EXCLUDE_DIRS for downstream projects

What does this MR do?

Our downstream test projects have paths that are filtered out by SAST_EXCLUDED_DIRS default value. This change overrides the default to help with testing.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/security-products/release/-/issues/98

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
  • [-] Auto-DevOps template
  • [-] Job definition example
  • [-] CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer