Rollup merge of #89789 - jkugelman:must-use-thread-builder, r=joshtriplett

Rollup merge of #89789 - jkugelman:must-use-thread-builder, r=joshtriplett

Add #[must_use] to thread::Builder

I copied the wording of the [`fmt::Debug` builders](https://doc.rust-lang.org/src/core/fmt/builders.rs.html#444).

Affects:

```rust
std/src/thread/mod.rs:289:5   std::thread::Builder   fn new() -> Builder;
std/src/thread/mod.rs:318:5   std::thread::Builder   fn name(mut self, name: String) -> Builder;
std/src/thread/mod.rs:341:5   std::thread::Builder   fn stack_size(mut self, size: usize) -> Builder;
```

Parent issue: #89692

r? `@joshtriplett`