dump rustflags too

Author: tgross35

patches/0001-Debug-builtins-output.patch

@@ -16,6 +16,7 @@ index 9721ddf090c..19a7886f496 100644
          let rustc_cfg = str::from_utf8(&out.stdout).unwrap();
 
 +        println!("cargo::warning=rustc: {}", env::var("RUSTC").unwrap());
++        println!("cargo::warning=rustflags: {}", env::var("RUSTFLAGS").unwrap());
 +        for line in rustc_cfg.lines() {
 +            println!("cargo::warning=cfg: {line}");
 +        }