Using Windows containers in Docker has been a painful experience. Slow
networks, no support for transparent networks due to the host being a
VPS, building large containers taking forever, the list goes on.
Meanwhile it costs me roughly €50 per month to keep the Windows CI
runner online.

Since we already have a Mac Mini at MacStadium (for free), we can just
re-use that Mac and run Windows inside a VirtualBox VM. This allows us
to work around the various Docker issues on Windows, and should reduce
our monthly infrastructure costs to more or less €0 (excluding the €1-2
I pay to AWS every month).

As part of this we also re-organise the runner tags a bit: the runners
now use three tags:

1. "inko"
2. "vbox" (to indicate it's a virtualbox runner)
3. "macos" or "windows", based on the type of OS

This ensures we don't accidentally pick up a shared runner that just
uses a "windows" or "macos" tag.