Steam Friends is missing menu functionality due to missing lsof package

**Game information**
It's not CSGO specific, but CSGO is a good example.

**Distribution name and version where applicable**

Arch Linux 5.14.8-arch1-1

**Flatpak info**
flatpak --version: Flatpak 1.11.3
flatpak --gl-drivers: default, host

**Problem description**

I don't see the menu to join a friend's game or see friend's game info when right clicking on the friend on the friends list (or friend's chat). When running steam with `flatpak -vv run com.valvesoftware.Steam` I can see this output once I click a friend with right click:

```
sh: line 1: lsof: command not found
```

**Does this issue reproduce with native Steam**

This issue is resolved on the native/pacman installed steam by installing the `lsof` package.