The plugin test was failing in a while as it depends on either presence of the software installed in the ci.jenkins.io nodes or an ability to install it while none of it is available there. The test should be rewritten (either in ATH or the plugin sources itself) using docker-fixtures (for example) to verify both scenarios (tools installed already, tools to be installed).

jglick, eitoball, are you interested in saving the test?

[1] https://github.com/jenkinsci/acceptance-test-harness/blob/7e8c9b20d25e420c37940313f8243e21c86be1bb/src/test/java/plugins/RvmPluginTest.java