libretro: Automatically download cores when not available

[Nilot352]

When a game is configured with it it asks for the runner to be downloaded which then simply does not happen.

[RobLoach]

https://github.com/lutris/lutris/blob/master/lutris/runners/libretro.py#L49

Looks like it's there....

Do you mean when a core isn't installed, it doesn't automatically download it?

[Nilot352]

Yes.

[strycore]

It depends on the cores, we do not ship all available cores and some have to be downloaded from Retroarch itself

[strycore]

Transferring the issue to buildbot