ci: install xvfb before Gradle build on self-hosted runners

Self-hosted Gitea Actions runners lack xvfb by default, so xvfb-run
fails with exit 127. Install it in the Java CI workflow before build.

.github/workflows/gradle.yml

@@ -23,6 +23,10 @@ jobs:
       uses: gradle/actions/setup-gradle@v5
       with:
         cache-cleanup: never
+    - name: Install Xvfb
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y xvfb
     - name: Build with Gradle
       run: xvfb-run --auto-servernum ./gradlew build --no-daemon --parallel
       env: