Disable cert test for non-default branches

What does this MR do?

Disable CI custom certs test for non-default branches. This currently prevents forks from having a passing pipeline

Alternatively we could set to allow_failure: true or perhaps set $CI_MERGE_REQUEST_PROJECT_PATH =~ /^gitlab-org/security-products/analyzers, but this was the quickest way to unblock

What are the relevant issue numbers?

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