Fix unexpected cfg condition name: ... warnings introduced in Rust 1.80

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

Author: hkratz

futures/build.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("cargo:rustc-check-cfg=cfg(futures_sanitizer)");
+}

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");
     }