These changes support incoming updates to generated coverage reports for
each separate library within the monorepo. Currently we can only get one
big coverage report for the whole repo, which provides a "false" view of
how well covered each individual library is. Unfortunately, the way
`--cov` needs to be changed (and to continue supporting ease of use for
developers), means some tool needs to be introduced to support the
dynamism. Tox happens to fit this use case.

This envvar is passed through without defining in the Tox config.