luac -l -l file.lua | awk "/_ENV/ { print $9 }" | sort -u | sed s/\"//g

only problem is (Busybox?) sed breaks pipes and redirection.

in the context of WoW, at least:

table.element is faster than table["element"]
local references to individual elements is faster than localizing a global table, that is:
local ref = _G.table.element
-- use ref
is faster than
local table = _G.table
-- use table.element