Hey! I'm a developer of the Godot engine (I'm on the Project Leadership Committee) and since you're using a completely unmodified version of the engine (3.2.3-stable.official) and this is a 2D game doing a normal Linux/X11 export should just work. In fact this is how I'm playing it now! (A user can also manually replace the .exe with the Linux version locally without needing the sources to the game)
It still might be nice to offer on itch for people who bought it and don't know this is easily possible?