zabbix: fix no-configure build variant

zabbix: fix no-configure build variant

When selecting only a package of "no-configure" build variant, e.g.
CONFIG_PACKAGE_zabbix-frontend-server=y
but not any other zabbix package, then the build fails.
The sources are not extracted and the install fails finally with:

make[4]: Entering directory '/srv/openwrt/openwrt-2.git/build_dir/target-arm_arm926ej-s_musl_eabi/zabbix-no-configure/zabbix-7.0.22'
make[4]: *** No rule to make target 'install'.  Stop.
make[4]: Leaving directory '/srv/openwrt.git/build_dir/target-arm_arm926ej-s_musl_eabi/zabbix-no-configure/zabbix-7.0.22'
make[3]: *** [Makefile:522: /srv/openwrt.git/build_dir/target-arm_arm926ej-s_musl_eabi/zabbix-no-configure/zabbix-7.0.22/.built] Error 2

This PR fixes this by always running the standard Prepare stage,
but skip the Install one when nothing needs to be compiled.

Signed-off-by: Michael Heimpold <redacted>