Rename backup file instead of copying

What does this MR do?

This MR builds on Include non-root user for spotbugs (!224 - merged) • Julian Thome • 17.6 and simplifies the file backup code to avoid permission issues, as explained here.

What are the relevant issue numbers?

Use non-root users in SAST analyzers (gitlab-org/gitlab#474602 - closed) • Julian Thome • 17.6 • On track

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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  • Job definition example
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• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer