Added a check for Steam installed as flatpak.

[fef-o]

Adding to Steam removes the arguments and uses "xdg-open" instead of "flatpak" in the .exe path when both Heroic and Steam are flatpaks.

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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)

[flavioislima]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[fef-o]

I have read the CLA Document and I hereby sign the CLA