Allow the plan to be overridden after setting the default

Instead of setting it from the environment and then reverting it back to
MMTk if it's still using the default plan.

gc/mmtk/src/api.rs

@@ -40,11 +40,12 @@ pub extern "C" fn mmtk_builder_default() -> *mut MMTKBuilder {
     let mut builder = MMTKBuilder::new();
     builder.options.no_finalizer.set(true);
 
-    // TODO(@mattvh): Clean up this hack
-    if matches!(*builder.options.plan, PlanSelector::GenImmix) {
-        let plan_selector = "MarkSweep".parse::<PlanSelector>().unwrap();
-        builder.options.plan.set(plan_selector);
-    }
+    // Set the default plan to MarkSweep
+    let plan_selector = "MarkSweep".parse::<PlanSelector>().unwrap();
+    builder.options.plan.set(plan_selector);
+
+    // And allow the environment to override it
+    builder.options.read_env_var_settings();
 
     // Between 1MiB and 500MiB
     builder.options.gc_trigger.set(GCTriggerSelector::DynamicHeapSize(1 << 20, 500 << 20));