fix incorrect signature in task::Builder::spawn_on

[TennyZhuang]

Signed-off-by: TennyZhuang zty0826@gmail.com

Motivation

All spawn* methods on task::Builder except task::Builder::spawn_on consume the ownership, and also task::join_set::Builder::spawn_on. I guess task::Builder::spawn_on accept the mutable reference is just a mistake.

Solution

Since it's an unstable API, break it to make all APIs more consistent is acceptable.

[Darksonn]

Thank you.

[Darksonn]

There's a rustfmt failure:

Diff in /home/runner/work/tokio/tokio/tokio/src/task/builder.rs at line 99:
     /// [runtime handle]: crate::runtime::Handle
     /// [`Handle::spawn`]: crate::runtime::Handle::spawn
     #[track_caller]
-    pub fn spawn_on<Fut>(
-        self,
-        future: Fut,
-        handle: &Handle,
-    ) -> io::Result<JoinHandle<Fut::Output>>
+    pub fn spawn_on<Fut>(self, future: Fut, handle: &Handle) -> io::Result<JoinHandle<Fut::Output>>
     where
         Fut: Future + Send + 'static,
         Fut::Output: Send + 'static,
Please run `rustfmt --edition 2018 $(git ls-files '*.rs')` to fix rustfmt errors.
See CONTRIBUTING.md for more details.

[ipetkov]

Refs #4850

[Darksonn]

Thanks!