nixos-rebuild: run systemd-run with / as the working directory

Before we would run systemd-run with the user's home dir as the working
directory, but this causes issues when the home dir is on a separate
partition and that partition cannot be mounted, as the transient unit
created by systemd-run would gain a dependency on the home mount and fail.
Since the root partition is guaranteed to be mounted, using that as the
working directory avoids this issue and allows doing a remote
nixos-rebuild to fix the issue preventing /home from being mounted.