Try to fix CI

Author: luke-gruber

test/ruby/test_transcode.rb

@@ -2391,8 +2391,9 @@ def test_ractor_asciicompat_encoding_doesnt_exist
       NO_EXIST = "I".freeze
       7.times do
         rs << Ractor.new do
+          no_exist = NO_EXIST.dup
           500.times do
-            if (val = Encoding::Converter.asciicompat_encoding(NO_EXIST)) != nil
+            if (val = Encoding::Converter.asciicompat_encoding(no_exist))
               raise "Got #{val}, expected nil"
             end
           end