EQL: Fix release build of EQL (#55113)

Enables EQL in release builds for testing

Fixes #55112

x-pack/plugin/eql/build.gradle

@@ -28,6 +28,9 @@ task internalClusterTest(type: Test) {
    * other if we allow them to set the number of available processors as it's set-once in Netty.
    */
   systemProperty 'es.set.netty.runtime.available.processors', 'false'
+  if (BuildParams.isSnapshotBuild() == false) {
+    systemProperty 'es.eql_feature_flag_registered', 'true'
+  }
 }
 
 check.dependsOn internalClusterTest