Remove --lib flag from prove6 usage

[ugexe]

Just saw this on the Rakudo Weekly News, nice work!

I noticed one thing that I think could be improved: remove the use of --lib with prove6.

From this:

    - name: Run prove6
      if: ${{ inputs.run_prove6 }}
      run: |
        zef install --/test --fetch-degree=${{ env.zef_fetch_degree }} --test-degree=${{ env.zef_test_degree }} App::Prove6

        prove6 --version

        prove6 --jobs ${{ env.zef_test_degree }} --lib --timer --verbose

To this:

    - name: Run prove6
      if: ${{ inputs.run_prove6 }}
      run: |
        zef install --/test --fetch-degree=${{ env.zef_fetch_degree }} --test-degree=${{ env.zef_test_degree }} App::Prove6

        prove6 --version

        prove6 --jobs ${{ env.zef_test_degree }} --timer --verbose

The reason being is the module and its dependencies have been installed, so there is no reason to add lib to the repository chain. More importantly though is that it will end up with the module being precompiled twice: once on the install, and once again when the lib directory gets used (which is different from what just got installed).