`build.rs` should not detect nightly feature

[loynoir]

quote

rust-lang/cargo#12872 (comment)

Personally, I think auto-detecting nightly features is a mistake and instead feature flags should be used to enable them explicitly.

actual

build.rs detect nightly feature.

thiserror/build.rs

Lines 12 to 13 in e9ea67c

	const PROBE: &str = r#"
	#![feature(error_generic_member_access)]

expected

build.rs should not detect nightly feature.

[loynoir]

Close as dtolnay not agree with epage.

dtolnay/anyhow#323 (comment)