onReady might be executed twice

If we call `ready()` in parallel, the `onReady` hooks are triggered twice

```js
import fastify from './fastify.js'

const app = fastify()

app.addHook('onReady', async function () {
  console.log('onReady')
})


await Promise.race([app.ready(), app.ready()])

await app.ready()
```

Note that this is also a problem when using `inject()`