This allows pkgs to be overridden in such a way that `<nixpkgs>` is
never imported, allowing home-manager to be used in environments where
`NIX_PATH` is not set.
PR #993
On non-x86 architectures (for example, aarch64) the installation of
home-manager fails indicating that it is attempting to select i686
packages for Linux and those aren't available.
Solution: make the condition for choosing these packages stricter