Offline installation (fixes VPN problem)

[DavHau]

Is your feature request related to a problem? Please describe.
It is currently not possible to use nixos-anywhere on hosts which run a software based VPN in order to be reachable, as that software will be killed by the kexec installer.

see #204

Describe the solution you'd like

Once the kexec installer is booted/executed, don't require the build host to re-connect, instead just execute the installation independently from the build host. Once the installation is finalized, the connection will be back due to the correctly setup nixos system.

Describe alternatives you've considered

Additional context

[Mic92]

Question is where it would the closures to install from? Also this way of installing seems a bit brittle, if you get the installation wrong than there is no way to recover from it.
Maybe it would be better to build a custom kexec image that is also able to connect from a VPN?