Remote access is now built into bitrise.io directly, no need to add any step: https://devcenter.bitrise.io/builds/remote-access/
It supports both SSH and VNC connection for Mac VMs (VNC is the protocol used by the macOS built in "Screen Sharing" app but of course you can find clients on other operating systems) so you can screen share into the macOS build VM (for Linux build VMs only SSH is supported at the moment).
I'll leave the original answer here as that still works:
There's a semi-official, ngrok based solution, using the https://github.com/bitrise-steplib/steps-remote-access-macos-ngrok bitrise step. You can find a setup & usage guide at: https://gist.github.com/viktorbenei/c6d4fe1e68de739dbb5f4f15de76b9db
A built in solution is also planned, in the meantime using ngrok
(which this unofficial step uses) or a similar tool can provide a solution.