Change default search engine

Hi,
I would like to change the default search engine via the configuration files.

I am able to modify other settings via `~/.var/app/io.gitlab.librewolf-community/.librewolf/librewolf.overrides.cfg`

But even if I add:

```
defaultPref("browser.policies.runOncePerModification.setDefaultSearchEngine", "searxng");
```

The settings is not picked up:


If I understand correctly that setting should be set in the `policies.json` but I cannot figure out where this file should be placed when using flatpak.

Any suggestions?

Thanks