Race condition in snaplist causing list of snaps to be (partially) empty

Because of a race condition, it is possible that the screen supposed to show the list of snaps shows an empty list of snaps (or partially empty).

The race condition is triggered when a network changed event (i.e., SNAPD_NETWORK_CHANGE) occurs while we are fetching the list of snaps.

The code vulnerable to the race condition is in this function from subiquity/server/controllers/snaplist.py:
```
    async def GET(self, wait: bool = False) -> SnapListResponse:
        if self.loader.failed or not self.app.base_model.network.has_network:
            await self.configured()
            return SnapListResponse(status=SnapCheckState.FAILED)
        if not self.loader.snap_list_fetched and not wait:
            return SnapListResponse(status=SnapCheckState.LOADING)
        await self.loader.get_snap_list_task().wait()
        if self.loader.failed or not self.app.base_model.network.has_network:
            await self.configured()
            return SnapListResponse(status=SnapCheckState.FAILED)
        return SnapListResponse(
            status=SnapCheckState.DONE,
            snaps=self.model.get_snap_list(),
            selections=self.model.selections)
```

And more specifically between the two following instructions:
```
        await self.loader.get_snap_list_task().wait()
        if self.loader.failed or not self.app.base_model.network.has_network:
```

When a SNAPD_NETWORK_CHANGE event is fired, the following happens:
 * the task that was returned by self.loader.get_snap_list_task() gets cancelled silently (without setting failed=True on the loader).
 * a new loader is created and replaces the old one: `self.loader = self._make_loader()`

Therefore, upon reaching the following instruction
```
        if self.loader.failed or not self.app.base_model.network.has_network:
```

, self.loader is now a new instance of SnapdSnapInfoLoader. That new instance is initialized with failed=False so the code does not go through the if body.

The first thing that comes to mind to fix this issue would be to do something like:

```
        loader = self.loader
        await loader.get_snap_list_task().wait()
        if loader.failed or not self.app.base_model.network.has_network:
```
but ideally, we would not consider this event an error and we would wait for the task of the new loader to finish.