Also limit build concurrency to 8.  A lot of the tests are I/O bound so they
are bottlenecked and extra cores don't help.

It was consistent so it worked fine but it was misleading since it points at
21.05, not the non-existent 20.15.

* 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?

don't like the duplication but not sure how to avoid it