Game initialization options not working

## To reproduce

1. Open game properties, "General" tab
2. Set initialization options to `some_command; %command%; some_command`, where `some_command` is an arbitrary command that works when executed in your terminal app.

## What happens
The game runs fine, but the commands don't execute.

## What I was expecting to happen
The command I'm specifying is supposed to turn off the Xfce compositor before the game starts, then turn it back on when the game exits.

```sh
xfconf-query -c xfwm4 -p /general/use_compositing -s false; %command%; xfconf-query -c xfwm4 -p /general/use_compositing -s true
```

Running this xfconf-query command on a terminal emulator works.

I've tested this on two games, Counter Strike Global Offensive and Counter Strike Source.

## System info
- Arch Linux (x86_64)
- Xfce 4.16 (started by xorg-xinit rootless)
- Steam installed as a flatpak from Flathub

Start at Steam's game properties > General > initialization options and reproduce the issue with the Flatpak on Arch Linux under Xfce. Trace how the semicolon-separated command containing %command% is passed to the game; done means commands before and after %command% execute around the game process.

Written by the indexing model from the issue text.