zabbix: fix agentd default user and minor issues

zabbix: fix agentd default user and minor issues

The last PR (https://github.com/openwrt/packages/pull/28370) missed
including two needed changes, and had a minor packaging Makefile
mistake.

The Zabbix Agent needs to drop privileges to the zabbix-agent user.
Similarly, if run as root (not the default), the Zabbix server needs to
drop privileges to the zabbix-server user.

There are also, in the Makefile, three instances of using BUILD_VARIANT
instead of VARIANT in package definitions.

So we fix those issues.

Signed-off-by: Daniel F. Dickinson <redacted>