fix skipped multi-user install steps on macOS

Some of the changes in #3788 to support non-systemd Nix installs
don't appear to be aware that the darwin installer exists, which
resulted in some skipped steps and inappropriate instructions.

scripts/install-darwin-multi-user.sh

@@ -37,6 +37,13 @@ poly_service_setup_note() {
 EOF
 }
 
+poly_extra_try_me_commands(){
+  :
+}
+poly_extra_setup_instructions(){
+  :
+}
+
 poly_configure_nix_daemon_service() {
     _sudo "to set up the nix-daemon as a LaunchDaemon" \
           cp -f "/nix/var/nix/profiles/default$PLIST_DEST" "$PLIST_DEST"