The test currently fails because we attempt to switch to a NixOS
configuration that is _very_ different from the one we are switching
from (e.g. the new configuration has an entirely empty /etc/fstab,
causing switch-to-configuration to want to start unmounting all
filesystems defined in the old configuration).
Test out both nix upgrade-nix and a NixOS upgrade.
Inject a fake fallback-paths.nix assuming a stable -> latest upgrade.
The NixOS upgrade does not use nixos-rebuild switch due to the
cost+annoyance of the instantiation needing
system.includeBuildDependencies.
