CI: fix new issues reported by luacheck 0.18.0 (#1383)

Fixes https://github.com/awesomeWM/awesome/issues/1380.

.luacheckrc

@@ -20,7 +20,6 @@ read_globals = {
     "drawin",
     "key",
     "keygrabber",
-    "mouse",
     "mousegrabber",
     "root",
     "selection",
@@ -30,9 +29,13 @@ read_globals = {
 
 -- screen may not be read-only, because newer luacheck versions complain about
 -- screen[1].tags[1].selected = true.
+-- The same happens with the following code:
+--   local tags = mouse.screen.tags
+--   tags[7].index = 4
 -- client may not be read-only due to client.focus.
 globals = {
     "screen",
+    "mouse",
     "client"
 }