Intermittent "feature cannot be used on stable release channel"

I temporarily got this error trying to build rustix as a dependency:
```
error[E0554]: `#![feature]` may not be used on the stable release channel
--> /Users/jyn/.local/lib/cargo/registry/src/github.com-1ecc6299db9ec823/rustix-0.36.4/src/lib.rs:99:26
|
99 | #![cfg_attr(rustc_attrs, feature(rustc_attrs))]
| ^^^^^^^^^^^^^^^^^^^^
```

I have unfortunately not been able to reproduce this error; I updated the version of rustix, which I originally thought fixed the issue, but when I downgraded to 0.36.4 I couldn't reproduce it again.

I think the build script needs to have `rerun-if-env-changed=RUSTC` or something like that? If it helps, my default toolchain is `nightly` but I have a `rust-toolchain` file that says to use 1.65.