Add downloading buildtools to workflow

.github/workflows/gradle.yml

@@ -18,5 +18,8 @@ jobs:
       uses: actions/setup-java@v1
       with:
         java-version: 1.8
+    - name: Download BuildTools
+      run: wget https://hub.spigotmc.org/jenkins/job/BuildTools/lastSuccessfulBuild/artifact/target/BuildTools.jar
     - name: Test with Gradle
       run: ./gradlew clean build -x test
+