All I had to do was run was `chmod -x ./Celeste64`. After that, you can either double click `Celeste64` in a file manager (if supported) or type `./Celeste64` in a terminal to start the game.
Installing the game systemwide might take more effort, since it seems like this was made to be a portable installation. Possible steps include:
Patching the source code to read resources from `/usr/local/share/celeste64/` instead of `./Content` and rebuilding the game
Moving the contents of `Content` to `/usr/local/share/celeste64/`
Moving the game executable to `/usr/local/bin/`
Ensuring all libraries are installed correctly (especially `libFosterPlatform.so`)
