Linux-Support for Unity Runtime

Although Mac and Linux are rare destination platforms for games, especially Linux is quite often used on build machines. Sadly the custom importer relies on the runtime, which isn't available for Linux at the moment. This forces us to a lot of efforts to provide an additional virtual machine, acting as a dedicated agent for project files, which contain the Rive runtime. It would be great, if at least cross build to windows would be possible on Linux, e.g., by providing only the necessary functionality.

Nevertheless, as we prefer to provide native builds for the Steam Deck, real runtime support would be the best. General native Linux builds are a rarity out there, but bypassing problems with the compatibitility layer for the Steam Deck is becoming a comon practice.