Specifying channel = "stable" in rust-toolchain.toml doesn't ensure the latest stable is used on BuildJet?

I got this error on `aarch64-unknown-linux-gnu` runner on BuildJet.

```
error: package `clap_derive v4.5.0` cannot be built because it requires rustc 1.74 or newer, while the currently active rustc version is 1.72.0
```

I placed the following `rust-toolchain.toml`, but it doesn't work probably because the rustup recognizes the version 1.72.0 is the "stable."

``` toml
[toolchain]
channel = "stable"
```

I know changing this to `nightly` or some specific version like `1.76` should work, but are there any handy way to use the latest stable toolchain?

The issue names rust-toolchain.toml, Rust channel selection, and an aarch64-unknown-linux-gnu BuildJet runner. Start by reproducing the reported rustc 1.72.0 behavior on that runner and tracing how the stable toolchain is selected. Done means establishing whether the configuration should select the latest stable toolchain and documenting or implementing the supported fix.

Written by the indexing model from the issue text.