[ci] Add 'main' builders in bringup mode (#4473)
[ci] Add 'main' builders in bringup mode (#4473)

Creates `main` versions of the existing `master` CI builders, as the
first half of switching LUCI tests over to the `main` branch.

Part of https://github.com/flutter/flutter/issues/90476
flutter/plugins,flutter/plugins,flutter/plugins,flutter/plugins,flutter/plugins,flutter/plugins,flutter/plugins,flutter/plugins