Fix backup functionality

[JimMadge]

:white_check_mark: Checklist

• [x] You have given your pull request a meaningful title (e.g. Enable foobar integration rather than 515 foobar).
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:vertical_traffic_light: Depends on

:arrow_heading_up: Summary

:closed_umbrella: Related issues

Closes #2270

:microscope: Tests

[github-actions[bot]]

Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
data_safe_haven/infrastructure/programs/sre
backup.py					47-55
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