Also setup Java 21 for Windows jruby-head build

Author: headius

.github/workflows/windows.yml

@@ -30,6 +30,12 @@ jobs:
         git config --global core.eol lf
         git config --global advice.detachedHead 0
     - uses: actions/checkout@v4
+    - uses: actions/setup-java@v4
+      with:
+        distribution: zulu
+        java-version: 21
+      if: >-
+        matrix.ruby == 'jruby-head'
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with: