Fix PipeSystem panicking with exclusive systems (#6698)

Without this fix, piped systems containing exclusive systems fail to run, giving a runtime panic.
With this PR, running piped systems that contain exclusive systems now works.

## Explanation of the bug

This is because, unless overridden, the default implementation of `run` from the `System` trait simply calls `run_unsafe`. That is not valid for exclusive systems. They must always be called via `run`, as `run_unsafe` takes `&World` instead of `&mut World`.

Trivial reproduction example:
```rust
fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_system(exclusive.pipe(another))
        .run();
}

fn exclusive(_world: &mut World) {}
fn another() {}
```
If you run this, you will get a panic 'Cannot run exclusive systems with a shared World reference' and the backtrace shows how bevy (correctly) tries to call the `run` method (because the system is exclusive), but it is the implementation from the `System` trait (because `PipeSystem` does not have its own), which calls `run_unsafe` (incorrect):
 - 3: <bevy_ecs::system::system_piping::PipeSystem<SystemA,SystemB> as bevy_ecs::system::system::System>::run_unsafe
 - 4: bevy_ecs::system::system::System::run

Author: inodentry

crates/bevy_ecs/src/system/system_piping.rs

@@ -83,6 +83,12 @@ impl<SystemA: System, SystemB: System<In = SystemA::Out>> System for PipeSystem<
         self.system_b.run_unsafe(out, world)
     }
 
+    // needed to make exclusive systems work
+    fn run(&mut self, input: Self::In, world: &mut World) -> Self::Out {
+        let out = self.system_a.run(input, world);
+        self.system_b.run(out, world)
+    }
+
     fn apply_buffers(&mut self, world: &mut World) {
         self.system_a.apply_buffers(world);
         self.system_b.apply_buffers(world);