This enables nushell integration by default for direnv, similar to
bash/zsh/fish. The slightly verbose way of setting this is to ensure
that peoples' existing nushell configuration isn't overwritten, only
appended to, as would be the case if we just used the integration
example from the nushell docs:
https://www.nushell.sh/cookbook/direnv.htmlCloses#3520
NixOS/nixpkgs@03310df843 disabled flake
support by default, so we now need to build a custom package and use it
if the user wants to `use flake` successfully. This should fix#2087.