apply: don't allow canceling during activation

[water-sucks]

Feature Description

Canceling an activation with Ctrl+C is bound to cause issues with atomicity of the currently running system.

nixos-rebuild already implements some protection of this using systemd-run, but this itself is not enough. We should also handle SIGTERM signals and force a second Ctrl+C signal before exiting the program.

Help

Yes

Issues

• I have checked existing issues and there are no existing ones with the same request.