fix unreachable macro calls for rust 2021 (#3889)

# Objective

- It was decided in Rust 2021 to make macro like `panic` require a string literal to format instead of directly an object
- `unreachable` was missed during the first pass but it was decided to go for it anyway now: rust-lang/rust#92137 (comment)
- this is making Bevy CI fail now: https://github.com/bevyengine/bevy/runs/5102586734?check_suite_focus=true

## Solution

- Fix calls to `unreachable`

Author: mockersf

crates/bevy_ecs/src/schedule/executor_parallel.rs

@@ -132,7 +132,7 @@ impl ParallelSystemExecutor for ParallelExecutor {
                             .finish_receiver
                             .recv()
                             .await
-                            .unwrap_or_else(|error| unreachable!(error));
+                            .unwrap_or_else(|error| unreachable!("{}", error));
                         self.process_finished_system(index);
                         // Gather other systems than may have finished.
                         while let Ok(index) = self.finish_receiver.try_recv() {
@@ -208,7 +208,7 @@ impl ParallelExecutor {
                     start_receiver
                         .recv()
                         .await
-                        .unwrap_or_else(|error| unreachable!(error));
+                        .unwrap_or_else(|error| unreachable!("{}", error));
                     #[cfg(feature = "trace")]
                     let system_guard = system_span.enter();
                     unsafe { system.run_unsafe((), world) };
@@ -217,7 +217,7 @@ impl ParallelExecutor {
                     finish_sender
                         .send(index)
                         .await
-                        .unwrap_or_else(|error| unreachable!(error));
+                        .unwrap_or_else(|error| unreachable!("{}", error));
                 };
 
                 #[cfg(feature = "trace")]
@@ -268,7 +268,7 @@ impl ParallelExecutor {
                     .start_sender
                     .send(())
                     .await
-                    .unwrap_or_else(|error| unreachable!(error));
+                    .unwrap_or_else(|error| unreachable!("{}", error));
                 self.running.set(index, true);
                 if !system_metadata.is_send {
                     self.non_send_running = true;