Merge pull request ruby#9077 from ruby/skip-only-gem-ractor-test

Skip Ractor related test with Windows platform

Author: hsbt

test/rubygems/test_gem_package_tar_header_ractor.rb

@@ -58,4 +58,4 @@ def test_encode_in_ractor
       assert_headers_equal header_bytes, new_header_bytes
     RUBY
   end
-end
+end unless RUBY_PLATFORM.match?(/mingw|mswin/)