Add cache cleaning task for ML snapshot (elastic#37505)

The ML subproject of xpack has a cache for the cpp artifact snapshots
which is checked on each build. The cache is outside of the build dir so
that it is not wiped on a typical clean, as the artifacts can be large
and do not change often. This commit adds a cleanCache task which will
wipe the cache dir, as over time the size of the directory can become
bloated.

x-pack/plugin/ml/cpp-snapshot/build.gradle

@@ -46,6 +46,10 @@ task downloadMachineLearningSnapshot {
   }
 }
 
+task cleanCache(type: Delete) {
+  delete "${projectDir}/.cache"
+}
+
 artifacts {
   'default' file: snapshotZip, name: 'ml-cpp', type: 'zip', builtBy: downloadMachineLearningSnapshot
 }