ci(workflow): enable datadog traces for the tests (#50619)
### What?
This PR attempts to enable datadog trace integrations to the next.js integration tests if env is configured. With this, datadog can observe each test suite's results and detect some meaningful information (i.e flaky) for us.
However, I wasn't able to verify this works with next.js repo since for some reason CI worker does not pick up the api key in the env (https://vercel.slack.com/archives/C04KC8A53T7/p1685597124894539). Still this won't affect existing workflow, and once enabled I can test it over vercel/turbo repo instead.
Partially resolved WEB-1150.
