Xcode 12 beta 6 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! Please note that the stack uses the iOS 13 SDK instead of the iOS 14 beta.
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/375/commits/b0b6178c7b511e24c5c82fea0f66cec43a92ef4b
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! 