nixos-install: Mound nixos and nixpkgs src directly

Previously nixos-install would fail if nixos and nixpkgs weren't on the same mountpoint as /

Signed-off-by: Shea Levy <shea@shealevy.com>
nixos-install: Include nix's perl libs in perl's INC

Signed-off-by: Shea Levy <shea@shealevy.com>