Carry through build.snapshot property on Scala builds

This commit also carries through the build.snapshot property when
forking the Scala build. This ensures that we are building snapshots or
not together with the rest of the artifacts.

Author: jasontedor

buildSrc/src/main/groovy/org/elasticsearch/hadoop/gradle/scala/ScalaVariantPlugin.groovy

@@ -46,6 +46,8 @@ class ScalaVariantPlugin implements Plugin<Project> {
                     properties.put('localRepo', 'true')
                 }
 
+                properties.put('build.snapshot', System.getProperty('build.snapshot', 'true'))
+
                 if (OperatingSystem.current().isWindows()) {
                     crossBuildForVariant.executable('gradlew.bat')
                 } else {