Fix #1558: add flake template for `nix flake init`
This depends on https://github.com/NixOS/templates/pull/40
This issue makes both nix run haskell-nix#hix -- init and
nix flake init --template haskell-nix#template --impure do the same
thing!
I should follow guidelines defined here https://nixos.org/manual/nix/unstable/command-ref/new-cli/nix3-flake-init.html#template-definitions in https://github.com/NixOS/templates/pull/40
Might be worth including the bash stuff in the docs in document tests. It's not actually testing anything in this repo except the documentation itself, but it would still be nice. Maybe we could also have it check that the flake.nix matches the hix init one?