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.

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.

that version we were on was pretty darn old, for sure

Co-authored-by: Tom Prince <tom.prince@hocat.ca>

We collect coverage info so we can't very easily run on multiple cores.  So we
mostly can't take advantage of the bigger machine.

With Cachix we can now cache even partial build successes so even if we lose
the whole cache and have to start from scratch, the build should now converge
on success even if it takes a few tries.

hope it does not break the build cachix setup step

Also move this time to a point shortly in my future so it will run and I can
see what happens.

Also, always store the cache because it should at least save us some effort
next time.