The test for command injection on Unix platforms should be omitted on Windows

ruby/ruby@c9ab8fe

Author: aycabta

test/test_rdoc_rdoc.rb

@@ -421,6 +421,7 @@ def test_remove_unparseable_tags_vim
   end
 
   def test_remove_unparseable_CVE_2021_31799
+    skip 'for Un*x platforms' if Gem.win_platform?
     temp_dir do
       file_list = ['| touch evil.txt && echo tags']
       file_list.each do |f|