build: fix build when gc is disabled

Change-Id: I8d3eb8874a4138668011b525c3b400a55a1f4866

src/libcmd/repl.cc

@@ -141,7 +141,11 @@ struct NixRepl
      * Note: This is `shared_ptr` to avoid garbage collection.
      */
     std::shared_ptr<Value *> replOverlaysEvalFunction =
+    #if HAVE_BOEHMGC
         std::allocate_shared<Value *>(traceable_allocator<Value *>(), nullptr);
+    #else
+        std::make_shared<Value *>(nullptr);
+    #endif
 
     /**
      * Get the `info` AttrSet that's passed as the first argument to each