fix(pageserver): compile warning of download_object.ctx on macos

[skyzh]

Problem

fix macOS compile warning introduced in 45ec868

Summary of changes

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]

2862 tests run: 2741 passed, 0 failed, 121 skipped (full report)

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Flaky tests (1)

Postgres 15

• test_vm_bit_clear_on_heap_lock: debug

Code coverage* (full report)

• functions: 28.0% (6604 of 23549 functions)
• lines: 46.8% (46982 of 100483 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
0b9d386 at 2024-05-02T19:38:41.382Z :recycle:}

[problame]

yuck

[problame]

Merged it because I actually need the build to work on my macOS :)