boulder/draft: Default to autotools on unhandled build system

This will most likely be the closest match, plus we write out the recipe
file unconditionally then much to deangst to our packagers.

boulder/src/draft.rs

@@ -54,9 +54,7 @@ impl Drafter {
         // Remove temp extract dir
         fs::remove_dir_all(extract_root)?;
 
-        let Some(build_system) = build.detected_system else {
-            return Err(Error::UnhandledBuildSystem);
-        };
+        let build_system = build.detected_system.unwrap_or(build::System::Autotools);
 
         let builddeps = builddeps(build.dependencies);
         let environment = build_system