Fix 404 when downloading Lua

See:

leafo/gh-actions-lua#49
leafo/gh-actions-lua#44

Author: rbong

.github/workflows/test.yml

@@ -9,11 +9,11 @@ jobs:
         include:
           - vim_type: "Vim"
             vim_version: "v8.2.4212"
-            lua_version: "luajit-2.1.0-beta3"
+            lua_version: "luajit-git"
             nvim: "false"
           - vim_type: "Vim"
             vim_version: "v9.0.1205"
-            lua_version: "luajit-2.1.0-beta3"
+            lua_version: "luajit-git"
             nvim: "false"
           - vim_type: "Neovim"
             vim_version: "head"