add java plugin to exercise, needed for zipTask

See: https://discuss.gradle.org/t/no-value-has-been-specified-for-property-archivefile-in-java-11-project/30348
for more info

Author: figaw

built-in-task-types/setup.sh

@@ -1,6 +1,10 @@
 #!/bin/bash
 
 read -d '' CONTENTS << EOF
+plugins {
+    id "java"
+}
+
 /*
 Available built-in types and documentation https://docs.gradle.org/current/javadoc/org/gradle/api/Task.html