refactor: work around the cfg

Co-authored-by: Alice Ryhl <aliceryhl@google.com>

tests-integration/tests/process_stdio.rs

@@ -236,8 +236,7 @@ async fn vectored_writes() {
     // on unix our small payload should always fit in whatever default sized pipe with a single
     // syscall. if multiple are used, then the forwarding does not work, or we are on a platform
     // for which the `std` does not support vectored writes.
-    #[cfg(target_family = "unix")]
-    assert_eq!(writes_completed, 1);
+    assert_eq!(writes_completed == 1, stdin.is_write_vectored());
 
     assert_eq!(&read.unwrap(), b"hello\nworld!\n");
 }