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.

pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh

@@ -898,7 +898,7 @@ elif [[ "$action" = switch || "$action" = boot || "$action" = test || "$action"
         "--no-ask-password"
         "--pipe"
         "--quiet"
-        "--same-dir"
+        "--working-directory=/"
         "--service-type=exec"
         "--unit=nixos-rebuild-switch-to-configuration"
         "--wait"