Pin Xamarin.Forms version used by EventBuilder to be same version as ReactiveUI.XamForms

[ghuntley]

Do you want to request a feature or report a bug?

bug

What is the current behavior?

EventBuilder is currently unpinned and always downloads the latest

This has resulted in a broken build

What is the expected behavior?

This needs to be pinned.

What is the motivation / use case for changing the behavior?

💚 builds

Which versions of ReactiveUI, and which platform / OS are affected by this issue? Did this work in previous versions of ReativeUI? Please also test with the latest stable and development snapshot

it's all on 🔥 - dev and master

[ghuntley]

Fixed by #1625

[ElijahReva]

Could it be the result of MSBuild update or something changed in CI environment? I also wasn't able to build locally even after pulling #1625.