I’m confused. If you’re trying to not let other developers download your game when developing, it’s impossible. The best thing you can do is make them develop outside of the game.
Allow contributors access to separate games with no major scripts or builds. That way, trusted developers can use their work without risking the main game being stolen.
Your best bet for this is what @bluebxrrybot advised you to do. Just like with any system, you should separate the jobs of each developer and only give each of them access to the specific thing they’re working on.
For example, if someone is a scripter and you want them to make you a system, replicate the system conditions to a different place and tell them to develop it there. If they’re a UI artist, do the same and only give them access to the UI interfaces they will be working on(basically copy them in a baseplate). If they’re a builder, give them the dimensions and configurations of what they should build, and any necessary parts, and tell them to build it in an empty place.
That way the only possible way for them to copy your game is if they somehow are able to collaborate behind your back without a single one of them reporting it to you. Also, you could switch your team once in a while, by hiring different scripters and builders for new things. That way you ensure a single scripter or builder won’t have access to many things at once, like many systems or many builds.