Don't install coverage while installing the package

[ptsavol]

• Instead, add a step to Github test runner action to install coverage

Re spine-tools/Spine-Toolbox#2268

Checklist before merging

• [x] Unit tests pass

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 61.68%. Comparing base (259f93c) to head (da925a3).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #202 +/- ## ======================================== Coverage 61.68% 61.68% ======================================== Files 190 190 Lines 17655 17655 Branches 2890 2890 ======================================== Hits 10890 10890 Misses 6183 6183 Partials 582 582 ```

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