Rust formatting broken due to apheleia change

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- [X] I have searched the [issue tracker](https://github.com/doomemacs/doomemacs/issues), [documentation](https://docs.doomemacs.org), [FAQ](https://docs.doomemacs.org/-/faq), [Discourse](https://discourse.doomemacs.org), and [Google](https://google.com), in case this issue has already been reported/resolved.

- [X] I have read "[How to Debug Issues](https://doomemacs.org/d/how2debug)", and will use it to provide as much information about this issue as possible.

- [X] The issue can be reproduced on the **latest** available commit of Doom.

- [X] The issue can be reproduced on a stable release of Emacs, such as 27, 28, or 29. *(Unstable versions end in .50, .60, or .9x)*

### Expected behavior

When trying to save a rust file in a normal Rust project (with edition 2018 or 2021), I'm expecting format-on-save to work correctly using either cargo fmt or rustfmt

### Current behavior

When I try to save a file, I get an error in the apheleia log buffer
```
$ rustfmt --quiet --emit stdout

error[E0670]: `async fn` is not permitted in Rust 2015
--> :54:1
|
54 | async fn main() -> anyhow::Result<()> {
| ^^^^^ to use `async fn`, switch to Rust 2018 or later
|
= help: pass `--edition 2021` to `rustc`
= note: for more on editions, read https://doc.rust-lang.org/edition-guide

Command succeeded with exit code 1.

```

### Steps to reproduce

1. Open emacs
2. Open a rust project
3. modify anything
4. save
5. observe the code not being formatted
6. go to apheleia logs

### System Information

https://pastebin.com/RDMQnN94

Start with the Apheleia log output and the Rust formatting setup used when saving a Rust file; compare the cargo fmt and rustfmt paths for projects using Rust editions 2018 or 2021. Done means format-on-save succeeds without invoking Rust 2015 behavior, and the result can be reproduced using the listed save steps.

Written by the indexing model from the issue text.