Automatically upload build reports as workflow artifacts

[bigdaz]

It's not unusual for workflows to contain a step following gradle-build-action to upload build reports as artifacts, like:

      - name: Store reports
        uses: actions/upload-artifact@v2
        with:
          name: linux-reports
          path: build/reports/

It would be nice to do this automatically (via opt-in parameter), with a sensible default path for build reports.