setup: update to use RbConfig

`Config` was removed years ago¹, and is not supported in any Ruby release
that dtas supports.

1. https://bugs.ruby-lang.org/issues/6809

1 files changed, 1 insertions, 1 deletions

diff --git a/setup.rb b/setup.rb
index 9f0c826..65380f4 100644
--- a/setup.rb
+++ b/setup.rb
@@ -785,7 +785,7 @@ class ToplevelInstaller
     else
       require 'rbconfig'
     end
-    ::Config::CONFIG
+    ::RbConfig::CONFIG
   end
 
   def initialize(ardir_root, config)