Skip a Ractor test unstable on Windows for MinGW

MinGW is also Windows, so it doesn't work for MinGW either.
https://github.com/ruby/ruby/actions/runs/17250269899/job/48950567246

Author: k0kubun

test/ruby/test_ractor.rb

@@ -164,7 +164,7 @@ def test_require_non_string
 
   # [Bug #21398]
   def test_port_receive_dnt_with_port_send
-    omit 'unstable on windows and macos-14' if RUBY_PLATFORM =~ /mswin|darwin/
+    omit 'unstable on windows and macos-14' if RUBY_PLATFORM =~ /mswin|mingw|darwin/
     assert_ractor(<<~'RUBY', timeout: 90)
       THREADS = 10
       JOBS_PER_THREAD = 50