Make demos work without colmena

[ibizaman]

It would be great to allow the demos to be deployed to without requiring to deploy through Colmena. Or at least keeping that optionally.

A few reasons:

• If we include all the configuration in configuration.nix, we can share the nix store between the VM and the host. This makes for much faster demos to run.
• Some users experienced issues when running build-vm-with-bootloader and could not run the demos.
• Would make Have a way to test demos in CI #53 possible.