[build] Use python3

This fixes building ffmpeg locally, where python is no longer available.

Run `fx format-code --files=run_yasm.py` while I'm here.

Change-Id: Ie20a7a3140af39e1468dd0ef006033c331535fa0
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/yasm/+/811950
Reviewed-by: Gulfem Savrun Yeniceri <gulfem@google.com>
1 file changed