cabal2nix-unstable is mostly used for regenerating the Haskell package
set. Thus we should aim to make it quick to rebuild in case its hash
changes because of Haskell related changes.
- cabal2nix is not fussy about the Nix version it uses for nix-env(1)
and we can just assume it is already in PATH like we do for
maintainers/scripts/haskell/*.
- nix-prefetch-scripts causes the most trouble since its python
dependencies depend on pandoc, so many Haskell changes require
an additional Python rebuild when building cabal2nix-unstable.
nix-prefetch-scripts is most likely installed and not necessary in
many cases, e.g. hackage2nix doesn't need them which is the main use
we have for cabal2nix-unstable. For the update scripts that do need
them, we add them to the used nix-shell explicitly.
Based on the Nixpkgs used and the version of nixfmt-rfc-style in that
version, it's likely that not the correct version is used.
Update scripts should instead run within a Nixpkgs development shell
(`nix-shell`/`nix develop`/`direnv`), where the correct version of
`nixfmt` (although `treefmt` should be preferred) is always available.
