build/base: fix the way to configure system-wild locales

In my last commit, I did know now this command would not work in a
chroot:
+ chroot /var/tmp/CI/all-redhat/install/RH7.0-1.7.0/base localectl
set-locale LANG=en_US.UTF-8
Failed to get D-Bus connection: Failed to connect to socket
/var/run/dbus/system_bus_socket: No such file or directory

So let's use another way to configure the locales like it's documented
in official manuals:
https://access.redhat.com/solutions/974273