build: luci.mk: honour PKG_VERSION if set
build: luci.mk: honour PKG_VERSION if set

Also pass correct variable to builds to ensure that version.lua and cache
buster strings are properly interpolated.

Signed-off-by: Jo-Philipp Wich <bd73d35759d75cc215150d1bbc94f1b1078bee01@mein.io>

lbthomsen/openwrt-luci,rogerpueyo/luci,openwrt/luci,rogerpueyo/luci,openwrt/luci,tobiaswaldvogel/luci,hnyman/luci,hnyman/luci,tobiaswaldvogel/luci,tobiaswaldvogel/luci,hnyman/luci,rogerpueyo/luci,lbthomsen/openwrt-luci,lbthomsen/openwrt-luci,openwrt/luci,lbthomsen/openwrt-luci,lbthomsen/openwrt-luci,openwrt/luci,hnyman/luci,tobiaswaldvogel/luci,tobiaswaldvogel/luci,rogerpueyo/luci,openwrt/luci,openwrt/luci,openwrt/luci,rogerpueyo/luci,tobiaswaldvogel/luci,tobiaswaldvogel/luci,hnyman/luci,lbthomsen/openwrt-luci,hnyman/luci,hnyman/luci,lbthomsen/openwrt-luci,lbthomsen/openwrt-luci,hnyman/luci,rogerpueyo/luci,rogerpueyo/luci,openwrt/luci,rogerpueyo/luci,tobiaswaldvogel/luci