dist: Rename Linux mime type to follow system convention

Mime type definitions are usually named like the mime type they define,
i.e. x-godot-project in our case.
It was introduced with reverse DNS naming as that's what FlatHub favors,
so it will need to be renamed there.

(cherry picked from commit 7eabf4c8a322a520c59d63b2bba36be44480101f)