create .DirIcon symbolic link

[ravener]

Continuing on ppy/osu#30759 I was waiting for Velopack to address this and they did at velopack/velopack#390

However, they decided to only do it for apps that use the default Velopack template, since osu! uses a custom AppDir, we'll need to comply with the AppDir spec on our own.

This creates a .DirIcon symbolic link to osu.png bringing us back to how appimagetool used to handle it before the Velopack migration.

I did end up using a symbolic link to follow appimagetool but Velopack seemed to have adopted a copy operation instead, just to note. I just felt like a symbolic link made more sense instead of duplicating files and to follow the original appimagetool