Add support for enabling CTU (--ctu) in clangsa analyzer

What does this MR do?

This MR introduces a new option to enable cross translation unit (CTU) analysis in the clangsa analyzer.

• Added a --ctu command-line flag.
• Added an environment variable ENABLE_CTU for enabling CTU.
• Updated buildAnalyzeArgs to append --ctu when enabled.

This makes it easier to configure CTU without requiring passthrough configuration.

What are the relevant issue numbers?

clangsa-sast: make the cross-translation-unit (... (gitlab-org/gitlab#570706 - closed) • Hua Yan • 18.5

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
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  • Job definition example
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• Ensure the report version matches the equivalent schema version
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer