clippy: ignore conditional drop-non-drop error

[ekzyis]

Without this commit, clippy without any flags fails:

error: call to `std::mem::drop` with a value that does not implement `Drop`. Dropping such a type only extends its contained lifetimes
   --> smite/src/scenarios.rs:167:5
    |
167 |     drop(runner);
    |     ^^^^^^^^^^^^
    |
note: argument has type `runners::StdRunner`
   --> smite/src/scenarios.rs:167:10
    |
167 |     drop(runner);
    |          ^^^^^^
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#drop_non_drop
    = note: `-D clippy::drop-non-drop` implied by `-D clippy::all`
    = help: to override `-D clippy::all` add `#[allow(clippy::drop_non_drop)]`

error: could not compile `smite` (lib) due to 1 previous error

This is the case because only NyxRunner implements Drop, so with --all-features, there's no error with clippy.

This is confusing Claude, and I think this would confuse new people, too. When you run cargo clippy and you get an error, it looks unintentional like a bug.