Summary:
This is currently breaking master, likely because of the Go 1.13 upgrade. With this upgrade it
seems that we need to explicitly pass in gcov flags.

Test Plan: bazel test //... and bazel coverage //src/...

Reviewers: oazizi, philkuz, #engineering

Reviewed By: oazizi, #engineering

Differential Revision: https://phab.corp.pixielabs.ai/D1916

GitOrigin-RevId: 83631e5bf6c51fdc78e6068f42635ee05991c4ba