Honor RUNTIME_JAVA_HOME for benchmarks (#28962)
Honor RUNTIME_JAVA_HOME for benchmarks (#28962)

With this commit we configure our microbenchmarks project to use the
configured RUNTIME_JAVA_HOME and to fallback on JAVA_HOME so this
behavior is consistent  with the rest of the Elasticsearch build.

Closes #28961
vroyer/elasticassandra,vroyer/elasticassandra,vroyer/elasticassandra