rb: just use @include("std")

we already have to use --override-std-dir
when running std tests, and having it this way
makes it much easier to run just the tests of this file.

lib/std/rb.zig

@@ -1,4 +1,4 @@
-const std = @import("std.zig");
+const std = @import("std");
 const assert = std.debug.assert;
 const testing = std.testing;
 const Order = std.math.Order;