Auto merge of #12953 - stanciuadrian:fmt, r=lnicola

Run stable `fmt` & `cargo` through `rustup`

`cargo test -p ide-assists` fails on Windows/x64/nightly:

```shell
> rustup self update
info: checking for self-updates
  rustup unchanged - 1.25.1

> rustup update
info: syncing channel updates for 'stable-x86_64-pc-windows-msvc'
info: syncing channel updates for 'nightly-x86_64-pc-windows-msvc'
info: checking for self-updates

   stable-x86_64-pc-windows-msvc unchanged - rustc 1.62.1 (e092d0b6b 2022-07-16)
  nightly-x86_64-pc-windows-msvc unchanged - rustc 1.64.0-nightly (affe0d3a0 2022-08-05)

info: cleaning up downloads & tmp directories

> rustup show
Default host: x86_64-pc-windows-msvc
rustup home:  C:\Users\stanc\.rustup

installed toolchains
--------------------

stable-x86_64-pc-windows-msvc
nightly-x86_64-pc-windows-msvc (default)

active toolchain
----------------

nightly-x86_64-pc-windows-msvc (default)
rustc 1.64.0-nightly (affe0d3a0 2022-08-05)

> cargo test -p ide-assists

test tests::sourcegen::sourcegen_assists_docs ... FAILED

failures:

---- tests::sourcegen::sourcegen_assists_docs stdout ----
thread 'tests::sourcegen::sourcegen_assists_docs' panicked at 'Failed to run rustfmt from toolchain 'stable'. Please run `rustup component add rustfmt --toolchain stable` to install it.', crates\sourcegen\src\lib.rs:141:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

failures:
    tests::sourcegen::sourcegen_assists_docs

test result: FAILED. 1576 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 1.82s

error: test failed, to rerun pass '-p ide-assists --lib'
```

After some investigation it seemed that [`cmd!`](https://github.com/rust-lang/rust-analyzer/blob/51705698bd66919435e4fcbc25d96bd7fc5583f4/crates/sourcegen/src/lib.rs#L139) didn't execute the expected (stable) rustfmt.

A simple `xshell` test failed too:

```rust
use xshell::{cmd, Shell};

fn main() {
    let sh = &Shell::new().unwrap();
    sh.set_var("RUSTUP_TOOLCHAIN", "stable");
    let version = cmd!(sh, "rustfmt --version").read().unwrap_or_default();
    println!("{version}");
}
```

Bypassing `xshell` and using `Command` directly failed too:

```rust
use std::process::{Command, Stdio};

fn main() {
    let child = Command::new("rustfmt")
        .arg("--version")
        .stdin(Stdio::null())
        .stdout(Stdio::piped())
        .env("RUSTUP_TOOLCHAIN", "stable")
        .spawn()
        .expect("failed to start");
    let output = child.wait_with_output().unwrap();
    let version = String::from_utf8_lossy(&output.stdout);
    println!("{version}");
}
```

Spawning `cargo +stable fmt version` [failed too](https://github.com/rust-lang/rustup/issues/3036) with `error: no such subcommand: +stable`.

Only `rustup run stable` worked fine for both `cargo` and `fmt`.

Thanks to `@lnicola` for a live investigation session, hints and tips.

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