Rollup merge of #138216 - Zalathar:any-debug, r=onur-ozkan

bootstrap: Fix stack printing when a step cycle is detected

When bootstrap detects a step dependency cycle (which represents a bootstrap bug), it is supposed to print out the contents of the step stack as part of its panic message.

However, while investigating #138205 it was found that bootstrap was actually printing out several copies of `Any { .. }`, because that is the Debug implementation for `dyn Any`. This is sadly not very helpful.

This PR fixes that problem by introducing a `trait AnyDebug: Any + Debug` that delegates to the underlying type's Debug implementation, while still allowing downcasting via Any.

---

The fixed behaviour can be verified manually (and is tested automatically) via a new dummy command, `./x run cyclic-step`:

```
$ x run cyclic-step
Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.02s

thread 'main' panicked at src/bootstrap/src/core/builder/mod.rs:1521:17:

Cycle in build detected when adding CyclicStep { n: 0 }
	CyclicStep { n: 0 }
	CyclicStep { n: 1 }
	CyclicStep { n: 2 }

note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Build completed unsuccessfully in 0:00:00
```

Author: matthiaskrgr