test_doc_comments fails if nightly is installed but isnt the default

[jayvdb]

---- tests::test_doc_comments stdout ----

thread 'tests::test_doc_comments' (2823998) panicked at src/lib.rs:150:51:
called `Result::unwrap()` on an `Err` value: Unstable("wrap_comments, normalize_doc_attributes")
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[jayvdb]

cargo +nightly test passes.

I am seeing this error occurring in my project after upgrading to Rust 1.93, when it is trying to use nightly rustfmt config. I am rechecking that it worked on Rust 1.92

[jayvdb]

installing latest nightly fixed my problem, so IMO rustfmt-wrapper should be trying to find the nightly rustfmt when it detects that Config.unstable() is true.

[jayvdb]

toolchain_find doesn't have a way to select the Rust channel, and I have not found an alternative that does this.