BREAKING CHANGE: this requires a newer version of `nix-shell`, at least
2.18 onwards. You can upgrade following the steps at:

https://nixos.org/download/

In particular, these are the bumped packages:

* GHC (9.4.8 -> 9.6.6)
* Cabal ( -> 3.12.1.0)
* Fortran ( 7 -> pinned)