Fix the if condition to skip test_ractor.rb correctly (ruby#13067)

Follow-up for ruby@a2b03ba

Author: ono-max

bootstraptest/test_ractor.rb

@@ -1636,7 +1636,7 @@ class C
 
   1_000.times { idle_worker, tmp_reporter = Ractor.select(*workers) }
   "ok"
-} if !yjit_enabled? || ENV['GITHUB_WORKFLOW'] == 'ModGC' # flaky
+} if !yjit_enabled? || ENV['GITHUB_WORKFLOW'] != 'ModGC' # flaky
 
 assert_equal "ok", %q{
   def foo(*); ->{ super }; end