Fix incompatibility with Lua 5.5

Under Lua 5.5, the control variable in `for` loops is declared `<const>`, which means attempting to assign to it causes an error. This results in a completely broken environment with an empty PATH. Per the reference manual, the correct fix is to define a local variable inside the loop with the same name as the control variable.

Author: MineRobber9000

hererocks.py

@@ -58,6 +58,7 @@
         end
 
         for path_part in (path .. path_sep):gmatch("([^" .. path_sep .. "]*)" .. path_sep) do
+            local path_part = path_part
             if path_part ~= hererocks_path then
                 if for_fish then
                     path_part = "'" .. path_part:gsub("'", [['\'']]) .. "'"