switch to v1-build on the nix ci

alex and happy aren't available for cabal v2-build in shellFor due to
haskell/cabal#8434
NixOS/nixpkgs#130556
NixOS/nixpkgs#176887

for some reason still needs `cabal update`, but doesn't download
anything from there

.github/workflows/nix-ci.yml

@@ -43,4 +43,5 @@ jobs:
     - uses: actions/checkout@v3
     - uses: cachix/install-nix-action@v22
     - uses: DeterminateSystems/magic-nix-cache-action@v2
-    - run: nix develop -Lv -c bash -c "cabal build"
+    # for some reason this doesn't work without update, even though it doesn't download anything
+    - run: nix develop -Lv -c bash -c "cabal update; cabal v1-build"

nix/shell.nix

@@ -3,11 +3,24 @@
   agda2hs-hs ? (import ./lib.nix { inherit pkgs; }).agda2hs-hs,
 }:
 pkgs.haskellPackages.shellFor {
+  # This doesn't result in a shell where you can use cabal (v2-)build,
+  # due to build-tool-depends in Agda's .cabal file, so for now only v1-build works
+  # Making cabal re-install alex and happy from Hackage can work,
+  # which will be done if the user runs `cabal update` and `cabal build`.
+  # relevant issues listed in:
+  # https://gist.github.com/ScottFreeCode/ef9f254e2dd91544bba4a068852fc81f
+  # main ones are:
+  # https://github.com/haskell/cabal/issues/8434
+  # https://github.com/NixOS/nixpkgs/issues/130556
+  # https://github.com/NixOS/nixpkgs/issues/176887
   packages = p: [ agda2hs-hs ];
-  buildInputs = with pkgs.haskellPackages; [
+  nativeBuildInputs = with pkgs.haskellPackages; [
+    # related to haskell
     cabal-install
-    cabal2nix
     haskell-language-server
+    # general goodies
     pkgs.agda
+    pkgs.nixfmt-rfc-style
+    cabal2nix
   ];
 }