Restore automated builds

[zimbatm]

Now that atlas is not taking care of the builds automatically for us anymore, we need a new way to automate the building of the boxes. The CI needs to support VirtualBox and ideally other hypervisors as well.

[griff]

MacOS runners for Github Actions has VirtualBox installed. Take a look at griff/nixos-vagrant which has automated builds and updates weekly. The builds on that project are very flaky though for some still undetermined reason.