When becoming other users, cd to /

When building '<nixpkgs>', nix-build checks to see if there is a file
literally called '<nixpkgs>' in the current directory (why?! it doesn't
use it if there is one).  If it can't access the current directory to
do this check, it fails with the error message:

  error: filesystem error: symlink_status: Permission denied [/home/alice/<nixpkgs>]

So we cd to a place where it can read the current directory.