This commit updates the CI to combine code coverage to a single file
and perform just a single upload after all tests finish (rather than
each test run uploading its own coverage report). This should reduce
failures / rate limiting on the codecov app, and also prevent the
codecov app from producing an inaccurate coverage report before all
the tests finish.

references:
- https://coverage.readthedocs.io/en/7.6.10/cmd.html#cmd-combine
- https://coverage.readthedocs.io/en/7.6.10/cmd.html#re-mapping-paths
- https://coverage.readthedocs.io/en/7.6.10/config.html#config-run-relative-files