* Update the readme to pull from cachix first, so users don't have to
build nix.
* Pull `nixFlakes` from override for slightly newer ui, and consistency
for users who change the nixos flake ref to something newer.
The bash wrapper was causing issues with deploy-rs and the
experimental-features option in the config was causing hercules-ci to
spit out a bunch of junk in the logs. Patching nix with the default
values required for flakes fixes both of these issues.