Set up Java 21 for jruby-head builds

Author: nobu

.github/workflows/macos.yml

@@ -31,6 +31,12 @@ jobs:
         git config --global core.eol lf
         git config --global advice.detachedHead 0
     - uses: actions/checkout@v7
+    - uses: actions/setup-java@v6
+      with:
+        distribution: zulu
+        java-version: 21
+      if: >-
+        matrix.ruby == 'jruby-head'
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with: