SDL: Force use DirectSound driver to fix wrong-pitch sound effects

This fixes the sound pitch bugs described in !373 (closed).

Context: SDL 2.0.6 switched to using WASAPI, which expects sound samples to be 32-bit floating point. We don't do that conversion in our sample handling, so this env var hack forces the old DirectSound driver.

It would be better to actually fix the sample handling, but this one-liner is quick and easy. I'm unaware of what benefits we would lose by doing this, because this is basically what the game was using before, with the 2.0.3 DLL.