add datadog env vars

[emmyoop]

resolves #8081 docs dbt-labs/docs.getdbt.com/# n/a

Problem

Datadog not ingesting test data for flaky test visibility

Solution

Adds env vars to turn on test visibility in datadog. Datadog docs

Updates the tox command to include the --ddtrace flag only when running in CI.

Pending

Update CircleCI: https://docs.datadoghq.com/continuous_integration/pipelines/circleci/

Checklist

• [x] I have read the contributing guide and understand what's expected of me
• [ ] I have run this code in development and it appears to resolve the stated issue
• [x] This PR includes tests, or tests are not required/relevant for this PR
• [x] This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

[mikealfare]

@cla-bot check

[cla-bot[bot]]

The cla-bot has been summoned, and re-checked this pull request!