fix: remove gitleaks archive after installation

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What does this MR do?

Delete gitleaks_${SCANNER_VERSION}_linux_x64.tar.gz archive in container image after gitleaks installation.

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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 updated/added for this feature/bug
• Job definition updated, if necessary
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• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer