fix(pageserver): allow unused_imports in download.rs on macOS (#8733)

## Problem

On macOS, clippy fails with the following error:

```
error: unused import: `crate::virtual_file::owned_buffers_io::io_buf_ext::IoBufExt`
  --> pageserver/src/tenant/remote_timeline_client/download.rs:26:5
   |
26 | use crate::virtual_file::owned_buffers_io::io_buf_ext::IoBufExt;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `-D unused-imports` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(unused_imports)]`
```

Introduced in #8717

## Summary of changes
- allow `unused_imports` for
`crate::virtual_file::owned_buffers_io::io_buf_ext::IoBufExt` on macOS
in download.rs

pageserver/src/tenant/remote_timeline_client/download.rs

@@ -23,6 +23,7 @@ use crate::span::debug_assert_current_span_has_tenant_and_timeline_id;
 use crate::tenant::remote_timeline_client::{remote_layer_path, remote_timelines_path};
 use crate::tenant::storage_layer::LayerName;
 use crate::tenant::Generation;
+#[cfg_attr(target_os = "macos", allow(unused_imports))]
 use crate::virtual_file::owned_buffers_io::io_buf_ext::IoBufExt;
 use crate::virtual_file::{on_fatal_io_error, MaybeFatalIo, VirtualFile};
 use crate::TEMP_FILE_SUFFIX;