cppClean task should remove old cppdistribution contents

The `git clean` we used to do at the beginning of the build used to take
care of this, but due to long paths on Windows we can no longer use this

Original commit: elastic/x-pack-elasticsearch@3210c7507b

build.gradle

@@ -47,7 +47,7 @@ project.ext.numCpus = Runtime.runtime.availableProcessors()
 
 task cppClean(type: Exec) {
     commandLine bash
-    args '-c', 'source cpp/set_env.sh && ' + make + ' clean'
+    args '-c', 'source cpp/set_env.sh && rm -rf cppdistribution && ' + make + ' clean'
 }
 
 task cppObjCompile(type: Exec) {