Luckily, just noticed that I introduced a bug as part of splitting up the original PR into two.

It wasn't going to have an impact until the package was properly installed from the store, but without the proper directory resolution in there (since the variable I removed is never defined now), in some scenarios it wouldn't be possible to launch Python from a process outside of the app package, such as a virtual environment.

Testing in a proper app container is difficult, but I'll see if I can come up with a way to do it. It might require a custom buildbot, so I'll see whether there's a way I can do that (historically we've not been able to wire internal work machines - including Azure hosted ones - into other CI systems, but I'll try again).