Fix render workflow to require Gradle

.gitea/workflows/build-readme.yml

@@ -24,6 +24,11 @@ jobs:
           distribution: temurin
           java-version: "21"
 
+      - name: Set up Gradle
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y gradle
+
       - name: Resolve latest Minecraft release
         run: |
           mkdir -p build