Feature: support env variables in known fixes

[arielj]

This PR adds support to define env variables in the known-fixes repo Heroic-Games-Launcher/known-fixes#6

The env variables are applied to the launch command if present in the fixes file.

I known that in that issue in the known-fixes repo there's a comment by @imLinguin about adding this information to ULWGL/UMU, but we still need to support Mac games or any env variable that could be specific for Heroic and not generic enough to add to UMU's database.

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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)