Fix build failure with minimal-versions

```
  error[E0433]: failed to resolve: could not find `Builder` in `async_task`
     --> src/lib.rs:219:44
      |
  219 |         let (runnable, task) = async_task::Builder::new().propagate_panic(true).spawn(
      |                                            ^^^^^^^ could not find `Builder` in `async_task`
      |
  help: consider importing one of these structs
      |
  87  + use crate::thread::Builder;
      |
  87  + use std::thread::Builder;
      |
  help: if you import `Builder`, refer to it directly
      |
  219 -         let (runnable, task) = async_task::Builder::new().propagate_panic(true).spawn(
  219 +         let (runnable, task) = Builder::new().propagate_panic(true).spawn(
      |
```

Author: taiki-e

Cargo.toml

@@ -16,7 +16,7 @@ exclude = ["/.*"]
 
 [dependencies]
 async-channel = "2.0.0"
-async-task = "4.0.2"
+async-task = "4.4.0"
 futures-io = { version = "0.3.28", default-features = false, features = ["std"] }
 futures-lite = { version = "2.0.0", default-features = false }
 piper = "0.2.0"