3ad8f52de0
since fc614c37c6 nixos needs access to its
own path (<nixpkgs/nixos>) to evaluate a system with documentation.
since documentation is enabled by default almost all systems need such
access, including the installer tests. nixos-install however does not
ensure that a channel exists in the target store before evaluating the
system in that store, which can lead to `path is not valid` errors.