test: mark Ruby 3.0 as experimental on windows

There are some failed unit test with Ruby 3.0 on windows,
Instead of omitting those test cases, it may be better
to mark "experimental" to clarify the issues.

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

.github/workflows/windows-test.yaml

@@ -13,9 +13,13 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby-version: ['3.0', '2.7', '2.6', '2.5']
+        ruby-version: ['2.7', '2.6', '2.5']
         os:
           - windows-latest
+        experimental: false
+        include:
+          - ruby-version: '3.0'
+            experimental: true
 
     name: Unit testing with Ruby ${{ matrix.ruby-version }} on ${{ matrix.os }}
     steps: