fix regressed build system unit test

lib/std/Build/Step/Options.zig

@@ -514,6 +514,7 @@ test Options {
             .result = try std.zig.system.resolveTargetQuery(.{}),
         },
         .zig_lib_directory = std.Build.Cache.Directory.cwd(),
+        .root_builder = undefined,
     };
 
     var builder = try std.Build.create(
@@ -523,6 +524,8 @@ test Options {
         &.{},
     );
 
+    graph.root_builder = builder;
+
     const options = builder.addOptions();
 
     const KeywordEnum = enum {