Enable build-dir layout v2 on nightly by default

src/workspace/features.rs

@@ -826,7 +826,7 @@ macro_rules! unstable_cli_options {
         /// Cargo, like `rustc`, accepts a suite of `-Z` flags which are intended for
         /// gating unstable functionality to Cargo. These flags are only available on
         /// the nightly channel of Cargo.
-        #[derive(Default, Debug, Deserialize)]
+        #[derive(Debug, Deserialize)]
         #[serde(default, rename_all = "kebab-case")]
         pub struct CliUnstable {
             $(
@@ -842,6 +842,21 @@ macro_rules! unstable_cli_options {
                 fields
             }
         }
+        impl Default for CliUnstable {
+            fn default() -> Self {
+                let mut unstable = Self {
+                    $(
+                        $element: Default::default()
+                    ),*
+                };
+
+                // Defaults to enabled on nightly unless explicitly opted out.
+                unstable.build_dir_new_layout =
+                    matches!(crate::version().release_channel.as_deref(), Some("nightly" | "dev"));
+
+                return unstable;
+            }
+        }
 
         #[cfg(test)]
         mod test {