Do not install Python packages in GitLab CI jobs

This MR needs to be merged after isc-projects/images!58 and isc-projects/images!57; merging those MRs and this MR needs to be done in lockstep or else something will break (merging only this MR will break precheck jobs for current "production" images; merging only the image MRs will break some system test jobs due to use of f-strings in existing system tests).