avm2: report more correct Capabilities.os

[Mesteery]

Not very accurate but probably better than nothing

[n0samu]

See also: #10469
I think it was never merged because we weren't able to agree on how much information we want to expose, how accurate it should be, etc :(

[Mesteery]

Yes I can understand. But some AIR applications rely on os for example, to find an OS-specific path, or use FileStream instead of URLLoader for macOS platforms (in case of Dofus), so for theses applications a correct os value is important and can possibly break the game if it's wrong

[evilpie]

We could do this for AIR only. I also think this changes thw OS for everything running on the web/extension to Linux, which we most likely don't want to do. Windows is a much much larger market.

[Mesteery]

What if we put Windows instead of Linux in the else?

[Mesteery]

Closing in favor of #17590