If we're a single-user installation of Nix, then we won't have root
superpowers to just ignore the permission bits. This means that we'll
need permission on the directory (if it's a directory) that we're moving
in order to move it with rename, because it must update the ".."
directory entry.
Fixes#4295.
This is already working in master, thanks to
e913a2989f (and followup cleanups).
2239953894