Suggestion: Allow for passing custom environment variables to only programs launched with BOX86/64
I have a usecase where I would like to launch a program (steam) with different environment variables for the x86 bins from the x86_64 bins. There is no builtin functionality to the steam launch script to support this (and it is difficult to modify without triggering an automatic reinstall).

What I would like is a way set environment variables for the boxes to pick up on and pass to any boxed binary.
something like
```
BOX86_ENV='DISPLAY=:2' BOX64_ENV='DISPLAY=:0' steam
```

of course other normal environment variables should continue to be passed as before to all binaries (like setting STEAMOS=1 and STEAM_RUNTIME=1)