Add coverage reporting from E2E testing

[StephenButtolph]

We should add coverage outputs from our E2E CI. go1.20 introduced the -cover flag to go build (ref) which dumps the coverage statistics after shutting down the process.

This should significantly improve our visibility into what is actually being hit during our E2E tests - which will allow us to better target improving our testing.

Note: we currently build using go1.19 (and will until go1.21 is released) but we could probably still use this to get coverage on demand (until we can provide coverage reports for every run)

[github-actions]

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