JAVA_HOME is not unset when using Functional Test Server

[tylersmalley]

In kbn-es, we unset JAVA_HOME to ensure the JDK with ES is used. In the functional

https://github.com/elastic/kibana/blob/master/packages/kbn-es/src/cluster.js#L283

This appears to not be the case when running with the Functional Test Server:

JAVA_HOME='/tmp' node scripts/functional_tests_server.js                                         SIGINT(2)  15.39 ⧖
 info Installing from snapshot
   │ info version: 8.0.0
   │ info install path: /home/tyler/elastic/kibana/.es/es-test-cluster
   │ info license: oss
   │ info Downloading snapshot manifest from https://storage.googleapis.com/kibana-ci-es-snapshots-daily/8.0.0/manifest-latest-verified.json
   │ info verifying cache of https://storage.googleapis.com/kibana-ci-es-snapshots-daily/8.0.0/archives/20200413-193954_79e50332206/elasticsearch-oss-8.
0.0-SNAPSHOT-linux-x86_64.tar.gz
   │ info etags match, reusing cache from 2020-04-14T17:42:51.632Z
   │ info install directory already exists, removing
   │ info extracting /home/tyler/elastic/kibana/.es/cache/elasticsearch-oss-8.0.0-SNAPSHOT-linux-x86_64.tar.gz
   │ info extracted to /home/tyler/elastic/kibana/.es/es-test-cluster
   │ info created /home/tyler/elastic/kibana/.es/es-test-cluster/ES_TMPDIR
 info Starting
   │ERROR could not find java in JAVA_HOME at /tmp/bin/java
   │

ES exited with code 1
    at createCliError (/home/tyler/elastic/kibana/packages/kbn-es/src/errors.js:21:17)
    at _outcome.exitCode.then.code (/home/tyler/elastic/kibana/packages/kbn-es/src/cluster.js:331:15)
    at process._tickCallback (internal/process/next_tick.js:68:7)

[elasticmachine]

Pinging @elastic/kibana-operations (Team:Operations)

[tylersmalley]

Related to #37316

@jbudz is there any reason to not make this the default universally?

[jbudz]

Resolved with #89437