Since we're currently on the unstable channel and will likely stay there
for a while, let's encode this in the update script.
Once we switch back to stable, if we do, we can still adjust it again.
This allows more flexible control over the working directory. Values for workingDir must always start
with a / and will then be relative to the repo root.
Previously this command upgraded to the latest unstable version of nixpkgs,
but this was often broken. Taking the latest stable branch should give
better results.
Wolfgang Walther