Nightly probe ignores CARGO_UNSTABLE_ALLOW_FEATURES

This is an alternative to `RUSTFLAGS="-Zallow-features="` that can also control cargo features. Compiling a crate that depends on anyhow with `CARGO_UNSTABLE_ALLOW_FEATURES= cargo +nightly build` will incorrectly try to use unstable features, resulting in a compilation error.

Start by reproducing the failure with `CARGO_UNSTABLE_ALLOW_FEATURES= cargo +nightly build` on a crate that depends on anyhow. Then locate the nightly probe logic and compare how it handles this variable with `RUSTFLAGS="-Zallow-features="`. Done means the empty variable prevents the probe from trying unstable features and the dependent crate builds successfully.

Written by the indexing model from the issue text.