There's this warning in the logs:

13:10:45,350 (main)  WARN Bootstrap:247 - future versions of Elasticsearch will require Java 11; your Java version from [/mnt/jenkins-workdir/tools/hudson.model.JDK/OpenJDK_8_Latest/jdk8u222-b10/jre] does not meet this requirement

We already have the necessary options in the Jenkinsfile to start Elasticsearch with a different Java home. Let's just make the ES Java home a constant and force it regardless of the tested JDK (default build, but also every other build that may start a local Elasticsearch instance).

Only necessary for Search 6.