CI coverage is flaky.

CI coverage is currently flaky, showing a lower percentage of checked in code.

In a PR (https://github.com/envoyproxy/nighthawk/pull/746) that had no code changes, the CI is indicating that we are under the 93.2% coverage threshold. This did not show up on the previous PR, and this PR has no code changes, so coverage value should not have changed.

Two separate CI runs both failed, but with different coverage numbers, indicating this flakiness:
93.0 - https://app.circleci.com/pipelines/github/envoyproxy/nighthawk/3178/workflows/ee4133ca-518d-4f2f-a84a-7e9beff3d1ee/jobs/29727
92.5 - https://app.circleci.com/pipelines/github/envoyproxy/nighthawk/3178/workflows/29bf16a1-7e31-4b19-b4f4-a7c432a52f6c/jobs/29730

To mitigate, we are temporarily reducing the coverage threshold.