The bug was probably exposed due to a change in path initialization (there's been a few changes here recently ;) ) that stopped normalising prefix. You can see that sys.executable in your dumped path has additional "." in the path, and that's never been the case before - they're always cleaned up earlier than that.

Adding in the appx _layout_ test (it's the whole test suite in an "installed" layout) was easier to do in CI than creating a buildbot config, so I threw it in as a test and then left it on. It's certainly not been failing this whole time.