ci: persist workspace further down the flow

[Jusshersmith]

Problem

With the introduction of circleci workflows, coverage reports were not being correctly found by the codecov script.

Solution

We persist to workspace further down the flow, after the point in which the test coverage file is created.

Notes

The Codecov script appears to exit gracefully to avoid failing builds - we can alter this behaviour with -Z:

-Z Exit with 1 if not successful. Default will Exit with 0