fix(pageserver): include aux file in basebackup only once

[skyzh]

Problem

Extracted from #6560, currently we include multiple copies of aux files in the basebackup.

Summary of changes

Fix the loop.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

3000 tests run: 2885 passed, 0 failed, 115 skipped (full report)

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Code coverage* (full report)

• functions: 32.7% (6913 of 21149 functions)
• lines: 50.0% (54180 of 108321 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
e50e6f5 at 2024-07-01T14:45:51.866Z :recycle:}