SpringBoot Document that SystemTestPlugin output should never be cached

Overview

Document build cache miss to ease detection of (new) performance regression.

Details

The systemTest task is being rerun systematically on CI https://ge.junit.org/c/tz35q4eso3ktg/we4fflr3ipauu/task-inputs?cacheability=cacheable&expanded=WyJpcGFwaG1zcmhwdDVlLXJvb3RzcGVjJDIkMSJd&task-text=shad#change-ipaphmsrhpt5e-rootSpec$2$1-0-0

Marking the task as non cacheable will prevent from analyzing the same cache miss in the future.