How can I specify the default OS of agent to launch on EC2?

[mattwomple]

I recently setup a private instance of WebPagetest on EC2 (using the AMI provided by the webpagetest team). For one of my EC2 regions, I specify the min/max instances as 1/2, respectively, so that an agent will always be available. Is it possible to specify that I want the linux agent auto-started instead of the Windows agent, so I can calculate lighthouse scores? Otherwise, I have to spin up the second instance right away.

Thanks,
Matt