Current the only stage 2 bootstrap leg in CI is run offline and is dependent on an offline stage 1 leg. This means if there are any prebuilts that don't exist in the referenced prebuilts tarball, the stage 2 bootstrap leg will not run (because the stage 1 leg will fail). This happens frequently during the preview timeframe. Because the stage 2 bootstrap leg is often not run, we don't surface issues as soon as they should (e.g. dotnet/installer#15532).

To solve this, we should add a stage 2 bootstrap leg that is based on an online stage 1 build.