fix: use correct display ID for WSL2 when setting up Xvfb

[TheSecurityDev]

WSL2 uses WSLg which only uses display :0

In order to get the UI screenshot tests running locally, I have to run the selfdrive/test/setup_xvfb.sh script first, but it just hangs on "Waiting for Xvfb..." without this fix.

Now it's still not a great experience because the screenshots are black, I think due to a fundamental WSL limitation, but at least the script runs and I can see the window.

Note: I specifically targeted WSL2 because it seems the display stuff is different on previous versions, but I have no way to verify so just leaving the behavior the same for those.