Steps to reproduce:

1. Copy MSI on host `C:\zabbix_agent2_plugins-6.4.20-windows-amd64.msi`
2. Open CMD as Administrator
3. Run `msiexec /i "C:\zabbix_agent2_plugins-6.4.20-windows-amd64.msi" /qn`
4. Check `C:\Program Files\zabbix\zabbix_agent2.d\mssql.conf`. The first variable has the wrong value "[ExeFile]"`

Result:

Contents of `C:\Program Files\zabbix\zabbix_agent2.d\mssql.conf`

### Option:Plugins.MSSQL.System.Path
#    Path to external plugin executable.
#
# Mandatory: yes
# Default:
Plugins.MSSQL.System.Path=[ExeFile]

As you can see it has a wrong value.

Expected:
When we run the installer not in hidden mode for example with `msiexec /i "C:\zabbix_agent2_plugins-6.4.20-windows-amd64.msi"` or directly from explorer, the content of `C:\Program Files\zabbix\zabbix_agent2.d\mssql.conf` is filled in fine:

### Option:Plugins.MSSQL.System.Path
#    Path to external plugin executable.
#
# Mandatory: yes
# Default:
Plugins.MSSQL.System.Path=C:\Program Files\Zabbix\zabbix-agent2-plugin-mssql.exe

So I assue it is a bug and the variable in the unattendend installation should be filled the same way as it is for the not hidden installation. 

I also have not found any kind of documentation for the plugins installer, maybe I'm just missing a flag?

I have attached you both MSI Installer log files.

Thank you!