Suppress nightly lint forget_copy in yoke_impls.rs

[sffc]

Fixes #3436

[sffc]

Hmm?

error: lint name `forget_copy` is deprecated and may not have an effect in the future.
 --> utils/zerovec/src/yoke_impls.rs:6:10
  |
6 | #![allow(forget_copy)]
  |          ^^^^^^^^^^^ help: change it to: `clippy::forget_copy`
  |
  = note: `-D renamed-and-removed-lints` implied by `-D warnings`

[sffc]

Feel free to merge once CI is green.