rules_scala coverage doesn't work with bazel 0.24.0 and later

```
bazel-0.24.0-linux-x86_64 coverage --extra_toolchains="//test/coverage:enable_code_coverage_aspect" //test/coverage/...
```

outputs an empty `bazel-testlogs/test/coverage/test-all/coverage.dat`

Start by running the Bazel coverage command shown in the issue against test/coverage/ and inspect the enable_code_coverage_aspect target and generated coverage.dat. Trace why the coverage output is empty with Bazel 0.24.0 or later; done means the test-all coverage.dat contains coverage data instead of being empty.

Written by the indexing model from the issue text.