fix: unbreak GitHub lua/test workflow

As noted here:

- leafo/gh-actions-lua#49

> It appears that LuaJIT.org has stopped publishing release tarballs on
> their website.[^tarballs]

ie.

> No Release Tarballs or Binaries
>
> LuaJIT uses rolling releases. There are no release tarballs available
> for download.

Suggested fix:

- leafo/gh-actions-lua#49 (comment)

> I just changed my workflow to use OpenResty's LuaJIT
> distribution[^OpenResty]. It's the same as the standard LuaJIT, which
> is based on Lua 5.1, except they've back-ported a few things from Lua
> 5.2 as well.
>
> Just change your `luaVersion` to `luajit-openresty`.

As the action states:

- https://github.com/leafo/gh-actions-lua/tree/master

> `luajit-openresty` — will allways pull master from
> https://github.com/openresty/luajit2

I'd rather not do this, because I'd rather test the same version that
Neovim embeds[^Neovim], but it looks like I don't have any choice right
now.

[^tarballs]: https://luajit.org/download.html
[^OpenResty]: https://github.com/openresty/luajit2
[^Neovim]: Via `:version`: LuaJIT 2.1.1725453128 (in Neovim v0.10.1).

.github/workflows/lua.yml

@@ -31,7 +31,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: leafo/gh-actions-lua@v10
         with:
-          luaVersion: luajit-2.1.0-beta3
+          luaVersion: luajit-openresty
 
       - name: Symlink luajit
         run: |