Programs run through protontricks don't seem to have access to game directories

Hi there - I'm trying to get Skyrim SSE set up with Mod Organizer in protontricks. However, when using Explorer in prontricks to set the path to the game, I discovered that it wasn't mounted anywhere (as far as I can tell). I couldn't manager to navigate to my steam folder and point MO to my Skyrim directory.

I'm using the Steam flatpak and the protontricks flatpak. Filesystem permissions have not been modified from the defaults, so `~/.var/app/com.valvesoftware.Steam` access remains.

Start by reproducing the issue with the Steam and protontricks Flatpaks using their default filesystem permissions, then inspect how programs launched through protontricks receive access to Steam game directories. Done means the relevant game directory is accessible without manually broadening permissions beyond the expected defaults.

Written by the indexing model from the issue text.