Move creation of /root to the activation script

...so it appears in a new installation before rebooting the system.

nixos/modules/config/users-groups.nix

···
           -I${pkgs.perlPackages.FileSlurp}/lib/perl5/site_perl \
           -I${pkgs.perlPackages.JSON}/lib/perl5/site_perl \
           ${./update-users-groups.pl} ${spec}
+
+        install -m 0700 -d /root
       '';
 
     # for backwards compatibility

nixos/modules/system/boot/stage-2-init.sh

···
 mkdir -m 01777 -p /tmp
 mkdir -m 0755 -p /var/{log,lib,db} /nix/var /etc/nixos/ \
     /run/lock /home /bin # for the /bin/sh symlink
-install -m 0700 -d /root
 
 
 # Miscellaneous boot time cleanup.