MBU - touch up linux support and add to CI

[thearst3rd]

This allows MBU to be built for linux, including compiling natively.

Unfortunately I realized there is currently an issue with multiplayer when running natively - If you a running the game natively and either join a server or somebody joins your server, the game crashes with this error:

Fatal glibc error: tpp.c:83 (__pthread_tpp_change_priority): assertion failed: new_prio == -1 || (new_prio >= fifo_min_prio && new_prio <= fifo_max_prio)
zsh: IOT instruction (core dumped)  LD_LIBRARY_PATH=/usr/local/lib ./marblegame

When running marblegame.hl with hashlink, the issue does not occur. So not sure what is up with that. That should be fixed before this is officially distributed.

Will target MBG and MBP after this. MBU was surprisingly nice with only one case-sensitivity related issue. I know MBP will be much more of a pita

[RandomityGuy]

Thanks for the PR, as for that specific error: https://github.com/godotengine/godot/issues/77818 Perhaps that may give an insight on fixing it. Seems like a datachannel bug