Rollup merge of #143180 - jieyouxu:forest, r=Kobzol

Use `tracing-forest` instead of `tracing-tree` for bootstrap tracing

I find the `tracing-forest` output easier to comprehend.

Note that this is not a strict improvement -- `tracing-forest` output contains some emojis and redundant log levels, but customizing it seems to be... non-trivial. Despite this, I still find `tracing-forest` easier to follow than `tracing-tree`, even when I tried to tune `tracing-tree` output.

### Preview

```bash
BOOTSTRAP_TRACING=debug ./x test library/std --dry-run
```

With `tracing-forest` (this PR), it looks like

![Screenshot 2025-06-29 174802](https://github.com/user-attachments/assets/1ab9150f-48f8-4fb3-b004-27bcf8bbc0f5)

With `tracing-tree` (before this PR), it looked like

![Screenshot 2025-06-29 175227](https://github.com/user-attachments/assets/659e254d-15ab-47dd-af11-98e2331d4d7a)

r? `@Kobzol`

Author: matthiaskrgr