Rust fix compilation for no_std targets #2 (#7553)

* Fix nightly no_std

* Fix nightly no_std

rust/flatbuffers/build.rs

@@ -0,0 +1,12 @@
+use rustc_version::{version_meta, Channel};
+
+fn main() {
+    let version_meta = version_meta().unwrap();
+
+    // To use nightly features we declare this and then we can use
+    // #[cfg(nightly)]
+    // for nightly only features
+    if version_meta.channel == Channel::Nightly {
+        println!("cargo:rustc-cfg=nightly")
+    }
+}