[ML] Make the URL of the ML C++ Ivy repo configurable (#48702)

At present the ML C++ artifact is always downloaded from
S3.  This change adds an option to configure the location.

(The intention is to use a file:/// URL to pick up the
artifact built in a Docker container in ml-cpp PR builds
so that C++ changes that will break Java integration tests
can be detected before the ml-cpp PRs are merged.)

Relates elastic/ml-cpp#766

x-pack/plugin/ml/build.gradle

@@ -13,7 +13,7 @@ esplugin {
 repositories {
     ivy {
         name "ml-cpp"
-        url "https://prelert-artifacts.s3.amazonaws.com"
+        url System.getProperty('build.ml_cpp.repo', 'https://prelert-artifacts.s3.amazonaws.com')
         metadataSources {
             // no repository metadata, look directly for the artifact
             artifact()