Fix release task not creating any artefacts

When the release plugin was upgraded, the configuration was not altered
to set a pre-release task needing executed, so no plugin JAR was being
created as part of the release step. The `assemble` task is now being
set to ensure the JAR exists in the relevant output directory for
dependent steps to pick up.

(cherry picked from commit 4aba3e1)

build.gradle

@@ -115,6 +115,7 @@ release {
         requireBranch.set('')
         commitVersionFileOnly.set(true)
     }
+    buildTasks.set(["assemble"])
 }
 
 tasks.jar.configure {