meh this is not great but tests.nix passes through a bunch of its args that
get captured in the args set... probably need to write and use set subtraction
to get them out, I guess.  First, see if this works though.

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.

as Tahoe now wants, so it can respect available space when allocating more for
a lease

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.

Defend against the length-contraction slot_testv_and_readv_and_writev DoS

Implement a minimal interface for reporting spent ZKAPs to a spending service.

Fix certificate validation errors during environment setup on CI

maybe this nix-env invocation will work

hopefully nix-shell behaves a little more predictably than nix-env wrt NIX_PATH

maybe 2.3.16, a very recent 2.3, will do better.