cfg: do not reconfigure if successfully initialized

The success include that cfg.variables.LUA should be found.

src/luarocks/core/cfg.lua

@@ -941,7 +941,7 @@ function cfg.init(detected, warning)
       return table.concat(platform_keys, ", ")
    end
 
-   cfg.initialized = true
+   cfg.initialized = exit_ok and cfg.variables.LUA ~= nil
    return exit_ok, exit_err, exit_what
 end