Ignore rm -rf failure

http://ci.rvm.jp/results/trunk-mjit@phosphorus-docker/4213386

It's failing with:
rm: cannot remove '/tmp/ruby/v3/build/trunk-mjit/tmp/test_rubygems_20220827-13666-ii8lcp': Directory not empty
rm: cannot remove '/tmp/ruby/v3/build/trunk-mjit/tmp/test_rubygems_20220827-13666-fy77y1': Directory not empty

I'd like to make sure the following `ENV.replace` is called and see if
there's any other issues.

test/rubygems/helper.rb

@@ -467,7 +467,7 @@ def teardown
 
     # FileUtils.rm_rf randomly fails on ci.rvm.jp trunk-mjit
     if ENV['RUBY_DEBUG']&.include?('ci')
-      system('rm', '-rf', @tempdir.shellescape, exception: true)
+      system('rm', '-rf', @tempdir)
     else
       FileUtils.rm_rf @tempdir
     end