Fix setting AtomicReferenceImplementation on TruffleRuby

* This accidentally worked before because until Ruby 2.7 `alias_method` returns the receiver.
* In Ruby 3 however, `alias_method` returns the first argument.

Author: eregon

lib/concurrent-ruby/concurrent/atomic/atomic_reference.rb

@@ -170,6 +170,7 @@ class TruffleRubyAtomicReference < TruffleRuby::AtomicReference
                                       alias_method :compare_and_swap, :compare_and_set
                                       alias_method :swap, :get_and_set
                                     end
+                                    TruffleRubyAtomicReference
                                   when Concurrent.on_rbx?
                                     # @note Extends `Rubinius::AtomicReference` version adding aliases
                                     #   and numeric logic.