Thanks for the report. I took a quick look.
We are installing adoptopenjdk-*-hotspot package https://github.com/travis-ci/travis-build/blob/0d3f1f908897605ec66c0d04b2e150741c4381dc/lib/travis/build/bash/travis_install_jdk.bash#L45-L57 I believe the expectation was that these packages would set necessary environment variables, but probably that was wrong. We will look into it.