sysbench commands should be executed with its own user && review lua scripts permission

There are two things we need to review:

1) Lua scripts uploaded by the charm should be read-only for any users, but only manageable by the charm itself
1.1) Copy the lua scripts to a separate folder that sysbench_svc can access, e.g. /var/lib/sysbench_svc
2) We must downgrade the permissions of sysbench. It must be executed with a `sysbench` user instead of `root`

Maybe consider a sysbench snap instead.

Start by reviewing how the charm uploads Lua scripts and invokes sysbench. Done means the scripts are placed where sysbench_svc can read them while remaining manageable only by the charm, and sysbench commands run as a dedicated non-root user; consider the snap option separately.

Written by the indexing model from the issue text.