Updates JS backend nix instructions

[dmjio]

Per feedback, we should ensure we're using miso's pinned nixpkgs for fetching ghcjs9122

[dmjio]

cc @akegalj

$ cachix use haskell-miso-cachix
$ nix-shell -p pkgs.pkgsCross.ghcjs.haskell.packages.ghc9122.ghc -I nixpkgs=https://github.com/alexfmpe/nixpkgs/archive/b594b289740a2bc917ed9c66fef5d905f389cb96.tar.gz

If you're on Darwin it might not be cached, so I will try to push that later today, otherwise that command should succeed (does on linux) barring any Darwin nix chicanery.

It's still a one-liner, so after this no nix should be required to use miso.

[akegalj]

Thanks, that works like a charm. By the way I used to have haskell-miso cache before

https://miso-haskell.cachix.org
miso-haskell.cachix.org-1:6N2DooyFlZOHUfJtAx1Q09H0P5XXYzoxxQYiwn6W1e8=

is that one still used or should I drop it in favour of haskell-miso-cachix ?

EDIT: I see its still uploaded to cachix, so its still probably used for some bits. Will use both.

[dmjio]

Yea both is fine, haskell-miso-cachix is used by CI now