style(test): Remove check-cfg warning

This is currently breaking CI because we run effectively `RUSTDOCFLAGS=-Dwarnings cargo +nightly doc`

Author: epage

crates/cargo-test-support/build.rs

@@ -1,6 +1,7 @@
 #![allow(clippy::disallowed_methods)]
 
 fn main() {
+    println!("cargo:rustc-check-cfg=cfg(emulate_second_only_system)");
     println!(
         "cargo:rustc-env=NATIVE_ARCH={}",
         std::env::var("TARGET").unwrap()