test: better isolate concurrent-install suite (#97546)
these were failing with 50% probability since today, for some reason:
https://app.datadoghq.com/ci/test/runs?query=test_level%3Asuite%20%40test.service%3Anextjs%20%40test.type%3Aturbopack%20%40test.suite%3Aconcurrent-install&agg_m=count&agg_m_source=base&agg_t=count&fromUser=false&start=1786528015972&end=1787132815972&paused=false
