Ensure /var/log/journal permission bits are set

Ensure permission bits are (re)set on each system activation with
explicit chmod call.

mkdir -m MODE PATH will only set the permission bits if PATH is
*created*, which means users that have old NixOS versions will continue
to have the old 700 permissions on /var/log/journal until they chmod
manually. With this commit the permissions will be set to 755 on system
activation.