Fix CI: unexpected cfg condition name: nightly

See https://blog.rust-lang.org/2024/05/06/check-cfg.html

Author: hkratz

futures/tests/no-std/build.rs

@@ -1,6 +1,7 @@
 use std::{env, process::Command};
 
 fn main() {
+    println!("cargo::rustc-check-cfg=cfg(nightly)");
     if is_nightly() {
         println!("cargo:rustc-cfg=nightly");
     }