Pass -Zunstable-args in case metadata doesn't

Author: jyn514

src/docbuilder/rustwide_builder.rs

@@ -583,7 +583,7 @@ impl RustwideBuilder {
         cargo_args.push("--verbose".into());
         // We know that `metadata` unconditionally passes `-Z rustdoc-map`.
         // Don't copy paste this, since that fact is not stable and may change in the future.
-        // Normally we would need -Z unstable-options, but we currently pass that too.
+        cargo_args.push("-Zunstable-options".into());
         cargo_args.push(r#"--config=doc.extern-map.registries.crates-io="https://docs.rs""#.into());
         if let Some(cpu_limit) = self.config.build_cpu_limit {
             cargo_args.push(format!("-j{}", cpu_limit));