style: allow unused ExecutorChild

[SteveLauC]

What does this PR do

Clear the following clippy warning:

error: field `0` is never read
   --> src/executor.rs:230:9
    |
[230](https://github.com/topgrade-rs/topgrade/actions/runs/9573017990/job/26393598748?pr=827#step:7:231) |     Wet(Child),
    |     --- ^^^^^
    |     |
    |     field in this variant
    |
    = note: `-D dead-code` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(dead_code)]`
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
    |
230 |     Wet(()),
    |         ~~

It is indeed unused, but we should not remove it.

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