it didn't fail before when building tahoe-lafs directly, I don't know
why (something else pulls in setuptools but it's unclear what).

it does fail without this when running the system tests at integration time...

They fail a lot so leave them disabled.

* switch from tahoe-lafs src on my laptop to github
* package required updated versions of tahoe-lafs dependencies
* use overlays to avoid version skew between different python libs
* copy/paste/edit the nixpkgs expressions instead of overrideAttrs because the
  latter doesn't converge, I guess?

Not currently working