Require python-pip for ghtools

This doesn't feel quite like the correct solution (surely setting `provider => pip` should make sure pip is available?), but it works.

    Error: Could not update: Could not locate the pip command.
    Error: /Stage[main]/Ci_environment::Jenkins_job_support/Package[ghtools]/ensure: change from absent to 0.21.0 failed: Could not update: Could not locate the pip command.