Upgrade to GHC 9.6.6 and upgrade pkgs.nix to use pinned 25.05
Eventually fixes #436 (closed).
I have to open this MR for now against #291 (closed) as it depends on the work I have done on removing accelerate-llvm
.
I have yet to test this on Darwin but I have managed to get a working nix-shell
on both Linux and Mac.