Re-enable coverage reporting.

Coverage was disabled because it started failing with the error:

```
Unable to read execution data file /home/runner/work/creek-system-test-gradle-plugin/creek-system-test-gradle-plugin/build/jacoco/test.exec
```

But only on build servers.

Though I was loathed to do it, I disabling coverage until I have time to investigate, as this is blocking builds for dependency updates etc. 