chg: [mariadb] prefer a symbolic link than a copy

This is to be consistent with ``mirror-dir`` executable installation, and
to avoid one step when updating VPS: you need only up-to-date ``0k-charms``
and no need to re-install host backup through this script to get update
of ``mysql-backup`` script.

Signed-off-by: Valentin Lab <valentin.lab@kalysto.org>

mariadb/hooks/install.d/60-backup.sh

@@ -52,7 +52,7 @@ fi
 
 
 apt-get install -y kal-shlib-{core,pretty,common} </dev/null
-cp -f resources/bin/mysql-backup /usr/local/sbin/mysql-backup
+ln -sf "${PWD}/resources/bin/mysql-backup" /usr/local/sbin/mysql-backup
 
 
 ##