toggling locales does not take effect

after upgrading to version 9.0.1, toggling the locale does not seem to cause a re-computation in the getters of my glimmer components that use the `moment` service.

example component and corresponding template:

```javascript
import Component from '@glimmer/component';
import { action } from '@ember/object';
import { inject as service } from '@ember/service';

export default class MyComponent extends Component {
@service moment;

/**
* the first day of the this week.
* is locale dependent, e.g. with "en" locale that value would be "Sunday"
* but with "es" locale it would be "lunes" (Monday)
*/
get firstDayOfWeek() {
return this.moment.moment().startOf('week').format('dddd');
}

@action
changeLocale() {
this.moment.setLocale('es');
}
}
```

```handlebars
{{this.firstDayOfWeek}}

Change Locale
```

i would expect that once i change the locale (invoking the `changeLocale()` action via a button click or the likes), that "lunes" would be returned from the getter, but it doesn't - it still gives me "Sunday".

this appears to be a regression, version 8 was working as expected.