Miscellaneous Windows tweaks

This MR makes the adjustments necessary to use the freshly provisioned Windows GitLab CI runner, gitlab-ci-10.lab.isc.org. After this MR gets reviewed, we will need to make a "hard switchover" that will require existing MRs to be rebased on top of this one before their CI pipelines will work on Windows again. This will need to happen within the next week.

Closes isc-projects/images#10