Ignore noisy rule

This lint will be triggered by a forthcoming change, see
https://github.com/fish-shell/fish-shell/pull/11990#discussion_r2455299865

It bans the usage of

```rust
fn hello() -> impl Debug {
    let useful_name = xxx;
    useful_name
}
```

in favour of

```rust
fn hello() -> impl Debug {
    xxx
}
```
Which is less humanly-understandable.

Part of #11990

Cargo.toml

@@ -162,6 +162,7 @@ rust.unknown_lints = "allow"
 rust.unstable_name_collisions = "allow"
 rustdoc.private_intra_doc_links = "allow"
 clippy.len_without_is_empty = "allow" # we're not a library crate
+clippy.let_and_return = "allow"
 clippy.manual_range_contains = "allow"
 clippy.needless_lifetimes = "allow"
 clippy.needless_return = "allow"