the branch has some python packaging fixes in nixpkgs that we need

We only have one version of tahoe-lafs here right now anyway, so the
identifier isn't ambiguous without a version number, and there are probably
more changes required to really support more than one version at a time.

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.

I dunno if this is the right solution but it makes the build work again.
Hopefully upstream will come up with a good fix of their own.

This requires a newer Tahoe to provide the `set_implicit_bucket_lease_renewal`
function.  Then update our wrapper to use it.  Also add a test for the
behavior.

There is an effect on mutables as well since Tahoe changed how you turn off
implicit lease renewal for them as well.  Just update to use the newer API to
keep things working as they were.