message: add push_str issue to trophy case
diff: @@ -269,6 +269,7 @@ Violations of [Stacked Borrows] found that are likely bugs (but Stacked Borrows
  * [`Vec::push` invalidating existing references into the vector](https://github.com/rust-lang/rust/issues/60847)
  * [`align_to_mut` violating uniqueness of mutable references](https://github.com/rust-lang/rust/issues/68549)
  * [Aliasing mutable references in `sized-chunks`](https://github.com/bodil/sized-chunks/issues/8)
  +* [`String::push_str` invalidating existing references into the string](https://github.com/rust-lang/rust/issues/70301)
  ## License
  