Drop a future in a way that doesn't offend clippy's delicate sensibilties

Hey, what's wrong with this code?

            debug!("Launching a circuit to test build times.");
            let _ = mgr.launch_by_usage(&usage, dirinfo)?;

That's right! We should be invoking await on the future returned by launch_for_usage!

Ordinarily the compiler would have warned us, but the let _ = told it to ignore the future. (We only wanted to ignore the result of the future.) Fortunately, Clippy in Rust 1.67 now warns about this.

(No, I didn't see the problem with the original code either.)

Diagnosis wrong, see below.