Doesn't start with init.lua

**Describe the bug**
When I use `init.vim` the preview starts as normal, and when I use `init.lua` nothing starts, with no errors.

**To Reproduce**
1. Install `vim-plug` and `paq.nvim` (sorry, apparently one can't use `vim-plug` with `init.lua` so I had to change when I rewrote it).

2. Take
```
" init.vim
set nocp
filetype plugin on
let g:NVIM_PLUGINS_DIR = stdpath('config') . '/plugged'
call plug#begin(NVIM_PLUGINS_DIR)
Plug 'iamcco/markdown-preview.nvim', { 'do': 'cd app && yarn install' }
call plug#end()
```
and
```
-- init.lua
vim.o.cp = false
vim.cmd("filetype plugin on")
require "paq" {
{"iamcco/markdown-preview.nvim", as="markdown-preview", run="cd app && yarn install" };
}
```
and install the plugin (with `:PlugInstall` for the first and `:PaqInstall` for the second).

3. Open some markdown file using `nvim -u init.vim file.md`. Run `:MarkdownPreview`. Works as expected (server starts at `localhost`, browser opens it).
4. Open the markdown file using `nvim -u init.lua file.md`. Run `:MarkdownPreview`. Nothing happens (and no errors). In particular, the server doesn't seem to have started (when I use the address I have when it works, I get "not found").

**Expected behavior**
Everything works exactly the same for the two setups.

**Desktop:**
- OS: Mac OS X 10.14.6

Log:

`checkhealth` gives:
```
health#mkdp#check
========================================================================
- INFO: Platform: macos
- INFO: Nvim Version: NVIM v0.6.0-dev+9-g490615612
- INFO: Node version: v16.4.2

- INFO: Script: std('data')/site/pack/paqs/start/markdown-preview/app/server.js
- INFO: Script exists: 1
- OK: Using node
```

Otherwise the plugin has been working great, thanks for all your hard work!