compute_ctl: Fix unused variable on non-Linux

[sharnoff]

Problem

./run_clippy now fails on non-Linux from an unused variable introduced by #7577.

See an example check-macos-build failure here: https://github.com/neondatabase/neon/actions/runs/8992211409/job/24701531264

Summary of changes

Add #[allow(unused_variables)] to the relevant function argument.

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]

3024 tests run: 2891 passed, 0 failed, 133 skipped (full report)

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Flaky tests (2)

Postgres 15

• test_detached_receives_flushes_while_being_detached[True]: debug
• test_vm_bit_clear_on_heap_lock: debug

Code coverage* (full report)

• functions: 31.2% (6256 of 20054 functions)
• lines: 46.7% (46935 of 100592 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
766f0da at 2024-05-07T22:45:11.555Z :recycle:}