Support Nix 2.20 before nixpkgs releases it

bump version to 0.6.4

v0.6.2: oops edition

version 0.6.1

Release 0.6.0

Version 0.5.10

v0.5.9: redo our nix env, support nix 2.17

0.5.8: Fix nix 2.14 support

It appears that the error builder is back, baby! So let's fix up that
conditional. Passes tests, should be right!

0.5.7: fix nix 2.13.1 compat

It seems that they reverted the error builder feature. Yowza.