Use ~/.local/share to store the game data

[Jujstme]

Is it possible, on linux, to set up the application so it uses a local directory inside the home folder?

The application bundled inside the flatpak is hardcoded to extract the game data into /var, which obviously works for flatpak itself but doesn't work if the application is run outside it (eg. repackaging).

The current AUR package for example edits the executable itself to write inside a .sunrdata directory inside the home folder.