I don't remember the file specifically, but steam uses a script to launch games if I remember correctly. That script sets up some library paths and other stuff that's needed, I remember editing that script a long while back to include either prime-run or gamemode, but I couldn't find any references to that script on Google, whenever I'm on the computer I'll try to find it.
Edit: I think the file is ~/.steam/debian-installation/ubuntu12_32/steam-runtime/run.sh
or similar, look for a run.sh
in some of the steam runtimes.