add Nix CI workflow; update Nix flake

.github/workflows/nix.yml

@@ -0,0 +1,21 @@
+name: nix
+
+on:
+  # run on every push to every branch (visibility doesn't matter)
+  push:
+  # don't want pull_request config, since it would run twice
+
+jobs:
+  nix-flake-build:
+    runs-on: ubuntu-latest
+    name: build (flake)
+    steps:
+    - uses: actions/checkout@v3
+    - uses: cachix/install-nix-action@v20
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
+    - uses: cachix/cachix-action@v12
+      with:
+        name: camfort
+        authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+    - run: nix build

flake.nix

@@ -8,18 +8,9 @@
     flake-parts.lib.mkFlake { inherit inputs; } {
       systems = nixpkgs.lib.systems.flakeExposed;
       imports = [ inputs.haskell-flake.flakeModule ];
-      perSystem = { self', pkgs, ... }: {
-        haskellProjects.default = {
-          # packages.example.root = ./.;  # This value is detected based on .cabal files
-          # overrides = self: super: { };
-          # devShell = {
-          #  enable = true;  # Enabled by default
-          #  tools = hp: { fourmolu = hp.fourmolu; ghcid = null; };
-          #  hlsCheck.enable = true;
-          # };
-        };
-        # haskell-flake doesn't set the default package, but you can do it here.
+      perSystem = { self', pkgs, config, ... }: {
         packages.default = self'.packages.fortran-src;
+        haskellProjects.default = {};
       };
     };
 }