Use RUNTIME_JAVA_HOME for benchmarks

[DaveCTurner]

Since #28071, we distinguish the JVM used for compilation (defined by JAVA_HOME) from the JVM used to run tests (defined by RUNTIME_JAVA_HOME, or JAVA_HOME if the previous is unset). However, it seems that the benchmarks still use JAVA_HOME: the benchmarks run in #28941 reported they were using my JDK 9:

# JMH version: 1.19
# VM version: JDK 9.0.1, VM 9.0.1+11
# VM invoker: /Library/Java/JavaVirtualMachines/jdk-9.0.1.jdk/Contents/Home/bin/java

even though Gradle reported that my runtime JDK is version 8:

  Gradle Version        : 4.5
  OS Info               : Mac OS X 10.13.3 (x86_64)
  JDK Version (gradle)  : Oracle Corporation 9.0.1 [Java HotSpot(TM) 64-Bit Server VM 9.0.1+11]
  JAVA_HOME (gradle)    : /Library/Java/JavaVirtualMachines/jdk-9.0.1.jdk/Contents/Home
  JDK Version (compile) : Oracle Corporation 9.0.1 [Java HotSpot(TM) 64-Bit Server VM 9.0.1+11]
  JAVA_HOME (compile)   : /Library/Java/JavaVirtualMachines/jdk-9.0.1.jdk/Contents/Home
  JDK Version (runtime) : Oracle Corporation 1.8.0_144 [Java HotSpot(TM) 64-Bit Server VM 25.144-b01]
  JAVA_HOME (runtime)   : /Library/Java/JavaVirtualMachines/jdk1.8.0_144.jdk/Contents/Home/

/cc @elastic/es-core-infra.