Merge branch 'makebuildserver-caching-fixes' into 'master'

./makebuildserver CI fixes

These are fixes for caching and flexible config related to the work of getting `./makebuildserver` running on Debian's and Guardian Project's Jenkins boxes.  @mvdan and I have been working with h01lger to get that going, here's the latest build log: https://jenkins.debian.net/view/reproducible/job/reproducible_setup_fdroid_build_environment_profitbricks3/lastBuild/console

See merge request !91