Restore lua environment persistence and update modified timestamp correctly

See #544

library/lua/dfhack.lua

@@ -444,21 +444,18 @@ function dfhack.run_script_with_env(envVars,name,...)
     local perr
     local time = dfhack.filesystem.mtime(file)
     if time == scriptMtime[file] then
-        f = scripts[file].runScript
+        f = scripts[file].__runScript__
     else
-        env = {}
-        setmetatable(env, { __index = base_env })
-        for x,y in pairs(envVars or {}) do
-            env[x] = y
-        end
         --reload
         f, perr = loadfile(file, 't', env)
         if not f then
             error(perr)
         end
+        -- avoid updating mtime if the script failed to load
+        scriptMtime[file] = time
     end
     scripts[file] = env
-    env.runScript = f
+    env.__runScript__ = f
     return f(...), env
 end