also refactor tests.nix and default.nix so they share more of the environment
setup which is now slightly more complex

it is nice to fail if someone passes in something that is not supposed to be
passsed in, rather than silently accepting and ignoring it

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.

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.

broken now because of dependency problems

(with Tom)

(with Tom)