[CI] security cli build failure

[costin]

Several builds in the CI have failed due to testingConventions target failing on 6.x (likely due to a backport):

Task :x-pack:plugin:security:cli:testingConventions FAILED

https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+6.x+matrix-java-periodic/ES_BUILD_JAVA=java11,ES_RUNTIME_JAVA=java8fips,nodes=virtual&&linux/195/console

https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+6.x+matrix-java-periodic/ES_BUILD_JAVA=java11,ES_RUNTIME_JAVA=java8fips,nodes=virtual&&linux/196/console

[elasticmachine]

Pinging @elastic/es-security

[dakrone]

I believe this was resolved in #38546