Game "Crea" needs Python 2.7 to work

**Game information**
The game is [*Crea*](https://store.steampowered.com/app/280520/Crea/), App ID 280520.

**Distribution name and version**
Fedora 33

**Flatpak info**
Version: Flatpak 1.10.1
gl-drivers: default, host

**Problem description**
The game shows as "running" for just a second or so, with no window ever showing. [Here is the output of Flatpak, Steam and the game](https://github.com/flathub/com.valvesoftware.Steam/files/5871211/flatpak.log). Search for `Starting app 280520` to find the start of the game output. Of note are the messages `Could not find platform independent libraries` and `Consider setting $PYTHONHOME`.

**Does this issue reproduce with native Steam**
It works fine with native Steam. [Here is the output of Steam and the game when it runs properly](https://github.com/flathub/com.valvesoftware.Steam/files/5871215/normal.log). Looking at the game's [/proc/\/maps](https://github.com/flathub/com.valvesoftware.Steam/files/5871218/crea-maps.txt) you can see that it's using the system's Python 2.7 installation.

**Further notes**
I tried running the game from within Flatpak by setting the game's launch options to `PYTHONPATH=/app/utils/lib/python2.7/site-packages %command%`, but that didn't help at all.

Start with the Flatpak, Steam, and game output in flatpak.log for App ID 280520, then compare it with normal.log and crea-maps.txt to trace the missing Python 2.7 runtime. Done means Crea launches and displays its window through Flatpak Steam on Fedora 33, rather than exiting immediately.

Written by the indexing model from the issue text.