Run buildtools so that we may access NMS.

This will need to be manually updated to use the current version supported by the plugin for the moment.

.github/workflows/maven.yml

@@ -25,7 +25,11 @@ jobs:
     - name: Set up JDK 8
       uses: actions/setup-java@v1
       with:
-        java-version: 8
+        java-version: 11
+        java-package: jdk # (jre, jdk, or jdk+fx) - defaults to jdk
+        architecture: x86 # (x64 or x86) - defaults to x64
+    - run: wget https://hub.spigotmc.org/jenkins/job/BuildTools/lastSuccessfulBuild/artifact/target/BuildTools.jar
+    - run: java -jar BuildTools.jar --rev 1.16.4
     - name: Cache Maven packages
       uses: actions/cache@v1
       with: