I was attempting to use Sauce to test a non-stock browser based on Chromium. To do do, I was installing that browser during the "prerun" phase. This non-stock browser also requires a custom chromedriver.exe.
It looks like the order of events for Sauce VM's is to start chromedriver/selenium before the "prerun" phase. Because chromedriver.exe is started with an explicit path to the browser binary, it refuses to start when the browser isn't yet installed at the specified path.
To be able to test this non-stock browser, I need the "prerun" phase to occur before chromedriver/selenium is started.