Allow empty /nix directory in multi-user installer

With macOS catalina, we can no longer modify the root system
volume (#2925). macOS provides a system configuration file in
synthetic.conf(5) to create empty root directories. This can be used
to mount /nix to a separate volume. As a result, this directory will
need to already exist prior to installation. Instead, check for
/nix/store and /nix/var for a live Nix installation.

scripts/install-multi-user.sh

@@ -330,7 +330,7 @@ EOF
         fi
     done
 
-    if [ -d /nix ]; then
+    if [ -d /nix/store ] || [ -d /nix/var ]; then
         failure <<EOF
 There are some relics of a previous installation of Nix at /nix, and
 this scripts assumes Nix is _not_ yet installed. Please delete the old