ci(fix): run luacheck on plugins dir

.ci/script.sh

@@ -6,4 +6,4 @@ source "${CI_DIR}/common.sh"
 travis_retry make fetchthirdparty
 make all
 make testfront
-luajit $(which luacheck) --no-color -q frontend
+luajit $(which luacheck) --no-color -q {reader,setupkoenv,datastorage}.lua frontend plugins

plugins/keepalive.koplugin/main.lua

@@ -18,6 +18,7 @@ local menuItem = {
     text = _("Keep Alive"),
 }
 
+local disable
 if Device:isKobo() then
     disable = function() UIManager:_startAutoSuspend() end
     menuItem.callback = function()