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