test MSVC build on GitLab CI

Here's a few patches to start testing the MSVC build on GitLab CI. This should prevent us from accidentally breaking it.

There's one not so great detail here: Something seems to be up with the GitLab runner, causing it to hang indefinitely while pushing out the resulting image. This does not happen when I do this locally, though, and I've pushed the image from my personal laptop instead for this to work for me. This is why this is marked as WIP, as this will probably bite us if we merge it without fixing this first.

Edit: This seems to work now. See here for a passing example.