FYI, publishing any game on Steam requires a $100 USD fee. It’s however refundable once the game reaches $1,000 USD in revenue. Since the Armory game would most likely be open source, the revenue would probably need to come from either sponsored advertisements or in-app purchases (perks, character items, etc.).
https://partner.steamgames.com/doc/gettingstarted/appfee
P.S.
In case you were unaware, this whole “open source community made-game” concept has already been thought of and discussed in the past. It didn’t exactly go anywhere, unfortunately.