Set Steam App ID variables when using the Runtime on Proton + fixup setting user-defined env vars on native games

[CommandMC]

• When the Steam Runtime is used for Proton, STEAM_COMPAT_APP_ID and SteamAppId are set to 0. This avoids a ProtonFix crash for GE-Proton
• User-defined environment variables are now set in setupEnvVars (they were in setupWineEnvVars before). Not sure why I did it that way back then. Because of that, setting these variables on native games works again now.

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Use the following Checklist if you have changed something on the Backend or Frontend:

• Tested the feature and it's working on a current and clean install.
• Tested the main App features and they are still working on a current and clean install. (Login, Install, Play, Uninstall, Move games, etc.)
• Created / Updated Tests (If necessary)
• Created / Updated documentation (If necessary)

[CommandMC]

With the latest commit, Proton's logging feature will now also work. Adding PROTON_LOG=1 to the environment variables will now give you a steam-0.log file in your home directory with the usual Wine log
Maybe we should even override the log folder (PROTON_LOG_DIR env var) and then append the log to our own game log automatically?

[CommandMC]

I'd say this is now feature-complete. I have not opted for automatically setting PROTON_LOG=1 since the log can get quite big, and it's not necessary to have it 90% of the time
If the user still needs the log, setting the variable and then navigating to the correct directory (/home/$USER/ normally, /home/$USER/.var/app/com.heroicgameslauncher.hgl/ for Flatpak) is fine

[flavioislima]

Looks nice 👍🏽