coverage_unit test broke after the refactor of test/run_envoy_bazel_coverage.sh

[jiajunye]

coverage_unit test broke after the refactor of test/run_envoy_bazel_coverage.sh in envoyproxy/envoy#39611.

Description:
test/run_nighthawk_bazel_coverage.sh is derived from test/run_envoy_bazel_coverage.sh over the Envoy repo. We need to refactor test/run_nighthawk_bazel_coverage.sh to accommodate the changes in test/run_envoy_bazel_coverage.sh introduced in envoyproxy/envoy#39611.

Reproduction steps:
Sync your local branch to Envoy envoyproxy/envoy#39611. Run the coverage_unit test locally, RBE or in CI.

ci/do_ci.sh coverage_unit

Logs:
https://dev.azure.com/cncf/envoy/_build/results?buildId=185186&view=logs&j=0e8f2c86-d75d-5b24-83e4-95d4e2e3dd73