Xcode 12 beta2 is now available! 
To select this Stack you just have to open your app on bitrise.io,
go to the Workflow tab (Workflow Editor), and on the Stack tab select the Xcode 12.0.x
option and your next build will start on the Xcode 12 stack.
Note: you can now select separate stacks for separate workflows!
Read more at: https://blog.bitrise.io/per-workflow-stack-selection-is-here.
You can find the system report here:
https://github.com/bitrise-io/bitrise.io/pull/365/commits/01520485c4752966fc8826e9da85001c7f522659
If you’d like to add additional tools to be pre-installed you can find the instructions on GitHub, for both the Linux and for the macOS stacks.
Information about Stack types & update schedules can be found here: discuss.bitrise.io/t/281.
Happy Building! 