allow opaque_hidden_inferred_bound warning on nightly (#2275)

Currently, our nightly builds are failing due to the new warning
`opaque_hidden_inferred_bound`, which triggers when an opaque type
(`impl Trait`) in an associated type position does not explicitly
include the associated type's trait bounds (e.g. returning a
`Service<Future = impl Send, ...>`) or similar.

Unfortunately, we cannot simply change our code to make the trait
bound's type explicit, as changing `impl Send` to `impl Future<...> +
Send` in this position results in a surprising error which I don't think
is correct:

```
error[E0277]: `impl std::marker::Send` is not a future
  --> linkerd/app/outbound/src/http/logical.rs:97:30
   |
97 |                     Future = impl Future<Output = Result<http::Response<http::BoxBody>, Error>> + Send,
   |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `impl std::marker::Send` is not a future
   |
   = help: the trait `futures::Future` is not implemented for `impl std::marker::Send`
   = note: impl std::marker::Send must be a future or must implement `IntoFuture` to be awaited
   = note: required for `stack::map_err::ResponseFuture<(), impl std::marker::Send>` to implement `futures::Future`

For more information about this error, try `rustc --explain E0277`.
```

See
#2268 (comment)
as well as the upstream Rust issue rust-lang/rust#107729, for details.

This should probably be reported on the Rust issue tracker, since a
warning that's (apparently) impossible to fix seems not great. However,
for now, we can simply allow this warning for our nightly builds.

This should fix CI.

.github/workflows/nightly.yml

@@ -13,7 +13,7 @@ on:
 env:
   CARGO_INCREMENTAL: 0
   CARGO_NET_RETRY: 10
-  RUSTFLAGS: "-D warnings"
+  RUSTFLAGS: "-D warnings -A opaque_hidden_inferred_bound"
   RUSTUP_MAX_RETRIES: 10
 
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